Why do I feel lightheaded all of sudden (today)

I was feeling fine a few days ago, well except the stress/anxiety and bowel problems I've been getting for the past couple months. But suddenly today, I don't feel so well and it's been a terrible day. Bad period cramps, loose stool, tiredness. I was supposed to hang out with my parents, not sit in the car or on the stupid toilet. I was sleeping on my car ride (idk why but I almost always sleep during a car ride) home and my mom told me to go sleep for a while if I feel tired. I stayed upstairs in my bed, playing some games (the new Granny game people are obsessed about). I felt even more tired and my head felt..weird? Like I should've rest but I didn't, just play games, freaking myself more, play games to calm myself down, google symptoms for the 50th time, freak out, repeat. I stood and immediately felt lightheaded and dizzy-almost? It went away within seconds and now, I generally feel not so well. Tired, not excessively but tired. Worried, of course. I got full pretty quickly, a bit constipated now. Straining but I can't poop. Some many people have commented on my posts and say the same thing, "You're fine, it's the anxiety". I feel better that people think I'm "fine". I believe them. When I'm calm, I actually feel fine. But my anxiety makes me think otherwise. I pretty sure everything is anxiety related but I can't stop it. I don't know why. I have had therapy for about a month, suddenly stopped for some reason. I learned to meditate, do yoga, exercise, do something fun, but my worries are sucking up my energy. Making me want to isolate myself and cry, wondering if I never had these mental issues, bowel issues, would I be happy? Like a teen girl should be? I don't know anymore, I need some comforting.

Forgot this but I'm like convinced that I have anemia or cancer or whatever. Most likely IBS and anxiety but again, my brain automatically goes to "YOU HAVE CANCER AND YOUR NEVER GONNA GRADUATE, GET A JOB, GET MARRIED, HAVE KIDS, OR GROW OLD".