Air ambulance Captain reported being notified that they had flown the helicopter in an unairworthy condition.

Narrative

Unbeknownst to me; I flew the helicopter in an unairworthy condition. Day 0 I pre-flighted the helicopter at XA:30 and did not find anything out of the ordinary. The med crew and myself completed three patient flights during that shift on Day 0. The shift began as normal. The mechanic had shown up just prior to our first flight-request to take out the dual controls after a week of flight training and to do an airworthiness check. He completed the airworthiness check prior to us lifting for the flight but did not have time prior to us lifting to take out the dual controls. Upon returning to base the mechanic was able to remove the dual controls and look at the stretcher. The med crew mentioned they had to manipulate the helicopter's stretcher hinges to get the wheels to drop and lock and that one of the wheels was loose. They asked me to ask the mechanic to take a look at the stretcher. The mechanic completed work on the stretcher prior to departing for the second flight.We completed our second flight of the day at XI:15 and received a third flight request just prior to XK:00 returning to base at XN:00. The next day; Day 1; I came in to pre-flight the helicopter at XA:30 and did not find anything out of the ordinary. We received a flight request at XL:00. After dropping off the patient (leg 3 of 4); the med crew came back to helicopter to load the stretcher and it was then the two crew members made a comment to me about the height of the loading deck in the aft. They mentioned they believed the deck was lower than normal and showed me in their opinion why they believed that to be true. I made a note of that. I then did an evaluation of the aircraft stance and completed 360 degree walk-around. In my opinion as the pilot in command (PIC); I did not see anything that would render the aircraft un-airworthy. We all proceeded to compete our walk-around and departed for base. We completed the flight at XP:30 and I passed down the aircraft and information to the night pilot. I told him to ask the med crew specifically about what they had told me about the aft deck sitting lower than normal.Day 4. I got a call from Company to discuss the circumstances around Aircraft X. They asked me to run them through the scenario of Day 0 and Day 1. It was during this conversation they told me that the helicopter had experienced a hard landing on the early morning of Day 0. It was at that time I was made aware that the four transports I had flown; post hard landing; were in an unairworthy aircraft even though myself or the crew members had no knowledge of the hard landing.Suggestions: Crew Resource Management. Communication between pilots during a pilot pass down brief regarding the events of the day would have rendered this aircraft un-airworthy prior to my 4 flights. Getting a mechanic to come look at an aircraft when a med crew member makes mention of an aircraft concern. Writing up crew member and pilot aircraft concerns prior to mechanic looking at an aircraft.