Air Carrier Flight Attendant reported feeling light headed and nauseous to the flight crew. After multiple updates to flight deck of the worsening cabin condition the flight diverted for medical assistance.

Narrative

Upon climbing on flight XXX me and the C FA (Flight Attendant) heard a loud hissing noise coming from the fwd entry door as well as the fwd service door. Shortly after hearing the noise both me and the C FA started feeling light headed and nauseous. I acknowledged the odd feeling and confirmed with the C FA the sensation which she replied 'I feel the same way' I contacted the B FA and asked him if he felt anything strange in which he replied 'I'm a bit nauseous'. I immediately contacted the Captain to verify that the status of the aircraft and to also let him know that crew was experiencing the following symptoms. The Captain told me that he had no warnings in the cockpit in which I replied 'okay if anything changes please notify us ASAP'.Five minutes pass after I hung up the phone and the symptoms had not only worsen but my judgement became impaired as well as my balance. I immediately use the fwd lav to vomit. I came out of the lav and instructed the C FA to use a POB (Portable Oxygen Bottle) because as instructed in recurrent training these are early signs of hypoxia. The C FA contacted the Captain yet again to inform him that our symptoms had worsen and passengers were beginning to complain about light headedness and headaches. I began to monitor the cabin in which I found extremely odd that 90 percent of the cabin was sleeping. A phenomena that's not heard of specially during a day flight. I put two and two together and began delegate with the crew that we were experiencing a slow decompression however the monitors in the cockpit did not seem to agree. I communicated with the Captain yet again in which he replied 'a lot of fumes came into the aircraft during take off that could explain your symptoms'. At this point it had been nearly 1.5 hours into the flight there was no reason for the 'fumes' to still be affecting us. As I attempted to do my onboarding service I could barely stand up straight and every few steps I felt as if I was gasping for air. The FO (First Officer) needed to use the restroom during my service in which I found it as an opportunity to speak with the Captain YET AGAIN regarding my symptoms as well as passengers on board. The Captain offered me the oxygen mask and told me that no bells were alarmed. As I breathed the oxygen in the cockpit I informed the Captain I was feeling better during the oxygen consumption. Little did I know the FO had taken it up to himself to walk to the aft galley and ask a passenger how he was feeling in which the passenger replied 'I'm not going to lie to you but; my head is playing games on me'. The FO informed the C FA that he had a strange sensation himself and that we would be diverting.Shortly after I came out the cockpit the haziness came back and the C FA informed me that she was feeling weak which at this point so was I. I made the executive decision for both of us to have a personal POB. Not five minutes pass by and the Captain tells me to secure the cabin as we were going to make a diversion into ZZZ1. I asked the B FA how he was feeling in which he replied 'fine'. As I'm doing my final walk through I collapsed and had to take a passenger seat and was unable to finish my walk through. I took a few breaths of the POB and ran to my jump seat. At this point I couldn't hold the weight of my neck any longer and was truly weak and slumping. the C says 'the only thing holding me up in the harness' in which I replied same; hang in there were almost there. Upon landing I open the entry door and was holding unto the bulkhead for support. Many passengers claimed to feel ill I was rolled away in a wheelchair and analyzed by medical professionals. The flight crew should have believed the cabin crew earlier on when the situation presented itself.