Robinson R44 helicopter pilot reported during liftoff at 50 feet AGL had to initiate an autorotation and land due to getting a low rotor horn and engine power dropping.

Narrative

I am a high time fixed wing ATP pilot that recently transitioned to rotorcraft and obtained my rotorcraft license. I flew to a location and landed in a yard the night before. In the morning; I offered to take passengers flying to breakfast. A local restaurant located on an airport had helipads out front; and gave us permission to land. While this was not the first time taking 4 adults in my R44; I will admit that I don't have much experience at heavier weights as most of the time I fly by myself or with one passenger. I did a weight and balance on Foreflight and asked for everyone's weight. I calculated that we were 15 pound under gross weight. Upon reaching the helicopter; I unloaded every bit of extra weight from under the seats and the back seats to be as light as possible. The landing area was on the top of a hill. There were rolling hills around; many of which were higher than my current elevation. I wouldn't say it was confined; but I had very little flat surface before the hills sloped down; with rising terrain in all quadrants. I followed the checklist and referenced the maximum manifold pressure that day; almost to max since we were nearly at sea level (around 1500ft MSL) and the temperature was quite high. On the conservative side; it showed 24.0 plus the 1.6 for takeoff; giving me around 25.6; which was very close to the red line at 26.3. In hindsight; the nearest local airport about 20 miles away was showing a density altitude around 2700ft. The winds were calm that morning; and I couldn't feel any wind. However; I was in a bowl.I did a full run up and normal pick up into the hover. I saw my MP (Manifold Pressure) was within the limits. I then started to apply forward cyclic to get through ETL (Effective Translational Lift) and reduce drag. The helicopter accelerated and 'shook' as I went through ETL. I kept raising the collective to climb but soon realized I wasn't going to out climb the terrain to the south. I did a modified quick stop maneuver to avoid the trees; and pulled into an OGE (Out of Ground Effect) hover and made a pedal turn to the right to avoid the trees. I likely was instinctively raising the collective at this time as well. Around this time; I got the low rotor horn and saw my engine power dropping to 95% or so. I thought I was having an engine failure so I initiated an autorotation and landed at the bottom of a hill into a nearby field. It was not a hard landing; although I did have some forward motion but not much. A very thorough inspection revealed no damage at all. I then shut down the helicopter and had the passengers walk to a nearby road to get out of the field. I started looking at everything from a mechanical standpoint since I'm a mechanic and have been maintaining the helicopter (under supervision of another Robinson mechanic). I checked the governor and engine and couldn't seem to find what the problem was. My front seat passenger had a video; although she only recorded the last 15 seconds of the flight. Some of the panel was visible in the video; although I couldn't see positions of the flight controls. I sent the video to my flight instructor; who called me to debrief and we think we know what happened.In the video; it was visible that my MP was beyond the red line limit. At this point I was looking more at terrain and not the panel; so I didn't notice. The engine and rotor RPM seemed to drop together on the dial as well. After the fact; I asked everyone for an honest weight and some were not being honest. I also found a pack of soda that was missed under one of the seats; and a fuselage cover that I did not account for. The new weight and balance showed me 15 pounds over gross weight. Additionally; the high density altitude; my inexperience; and likely continuing to pull collective all contributed to a low rotor situation. I incorrectly saw this as an engine failure. The only time I have heard the low rotor horn in my 100 hours or so in helicopters is during autorotation training. For me; the horn has always signified engine failure. Additionally; although I have had training with low rotor recognition and recovery; it was only a handful of times and the muscle memory to lower the collective and roll the throttle on (overpowering a governor) was not present. This is probably one of the downsides to flying only helicopters with a governor. Right after getting my license a few months ago; I went and did about 20 full down advanced autorotations with an instructor. I attribute the success of this day's maneuver to that training; and some luck that where I landed was fairly level (although rough and uneven) and I didn't roll over.I attribute a lot of this to the Dunning-Kruger effect. I am very comfortable in aircraft and especially off airport operations from flying bush planes. I overestimated my skillset after getting my license; and have been operating at the maximum limits of my helicopter despite having very little rotorcraft experience. To avoid this in the future; I am taking a step back; getting more training; and limiting myself to 2 passengers and operating 150 pounds under gross weight until I gain experience. I will also be mindful of density altitudes in the future.