Welcome to the forum!
Gosh, I'm sorry for your troubles.
Yes, weight loss of 10 lbs or more is very common with flaring. Once you get it under control you'll stop that dramatic "losing" thing.
My son was diagnosed at 13, and he had to do prednisone. His colon was a mess, and it seemed like the best option to us. He also started Asacol, which he still takes. He was on and off the prednisone within 4 months. Yes, he did gain weight, but at that age he wouldn't listen to reason about
what to eat when he got the "steroid munchies!" If one eats lower calorie foods and stays away from salty snacks, that weight gain can be less than if you give in to the urge to have burgers and chips constantly!
Your diet sounds like the exact thing many of us have discovered - certain foods make the symptoms worse while we're in a flare. Can you eat cooked fruits and veggies.....? I can eat a lot of things as long as they're cooked soft.
Some of us find that diet makes a huge difference in overall UC (frequency of flares, severity of flares) but others don't. I am one that does fine on a pretty non-restrictive diet - although, because I've had UC for almost 24 years, I have sort of adapted what I think is "normal" to eat. When I'm not flaring, I can eat anything I want - salads, etc.
Are you in school? How's that going? Do you have a 504 plan in place? (If not, I'll be glad to share our experience with that.)
I would suggest a probiotic. My son had a little flare this spring and it responded beautifully to 10 days of an antibiotic (flagyl), upping his asacol by one pill, and adding a probiotic.... also, we added a fiber supplement, which often in our case clumps the poops together (even if the rest of the world thinks of them as laxatives....lol.)
To answer your questions, the side effects of being on prednisone for a long time can be nasty, contributing to osteoporosis, etc - but if we're on it not too long, it's the immediate side effects that are obnoxious. But, it does help a lot of people get unflared relatively fast.
If you haven't yet, check out ccfa.org and the "kids and teens link". there are some good tips, even the small things, like carrying your pills in a mint tin so you don't feel so weird having them on you.
I hope you get out of this flare asap, and---- ask any questions!