How to proceed after you realized you’re claustrophobic?

I risk to sound stupid, but I think I’ve suddenly realized I feel afraid in tight spaces. I don’t know what changed. This never used to be a problem before. Maybe when I was younger, I was smaller and somehow such places didn’t seem tight? Or maybe I knew less when younger and had more optimistic views? Not sure. All I know is I can’t handle being in rooms with no exit. It’s not just a feeling of unease or feeling uncomfortable… I feel dread. My lungs begin to fail me, and I sense a rising sense of panic. The problem is I’m not talking about the obvious fears like elevators, those can be avoided by taking the stairs. Recently I’ve caught myself being anxious when I am in closed spaces with a lot of people! The first time it happened, I was in an underground nightclub with two of my friends. The place is popular, many people visit it every night. And I think I was the first person in history to almost faint shortly after we arrived there. I felt nervous the moment we descended a narrow staircase, maybe because the club was packed with people, and I don’t like such places. While my friends were having their first round of shots, my brain was calculating how fast this place would get on fire and how many people would be able to get out at once using that staircase! I convinced myself that it was the smell of cigarettes and sweat that went to my head and made me dizzy. But how can I explain that I get the shivers when my niece watches a scene where the main character finds himself inside the belly of a monster?? Me, the 25-year-old grownup, has to turn away to stop my mind from picturing myself inside that slimy bag of skin! My last straw was my panic in the theater on Saturday night that almost made my fiancée call the ambulance… I think I fear my sense of fear more than anything else now! We were sitting in the middle, with narrow rows of seats and a palpable sense of closeness to other people in front of me and behind. All my mind could think of was this crowd, and the distance to the closest exit. When the lights dimmed and the person to the right from me took his seat, I went off the rails… The urge to flee, to escape the confinement of the room was more than any desire to see the show. My fiancée said he saw the blood literally drain out of my face. What do I do now? Do I have to avoid crowded events now, and meticulously plan my routes to ensure escape?? What if I begin to fear flying? After all, it’s technically sitting with a bunch of people in a metallic box in thin air. This is crazy. 😫