As you can read you are not too young! I was diagnosed in my mid-40's, my age is not the typical either. However, when I was taking minocycline for my mid-life acne...that gave me this disease.
A sigmoidoscopy is a shorter scope through the rectum up to the sigmoid part of the colon....basically left sided colon. Not sure why they want to do that if you already had 2 colonoscopies. Making brief adjustments to your diet may help with the pain, like a low reside diet....basically no fiber foods. Did they give you a blood test to test if you have UC for sure and not Chrons? I was on Lialda (another 5ASA)for about 1 year, took 4 tablets a day (the max) It helped, but not completely, now I just started Remicade (still on loading dose), my symptoms have improved greatly and off of prenisone. You maybe ready for the stronger meds the "big guns" like Remicade or Humira. Remicade has been approved for UC, but Humira is not yet so insurance probably will not pay for Humira unless you fail with the Remicade and jump through many hoops. Humira will be approved probably within 2 years (the FDA is soooo slow). MD's will start out with the main stay meds before they treat with the stronger meds. This is a frustrating disease because everyone is sooo different, but if you do your part, eat the foods that works for you, (like no raw vegs, no carbinated drinks, fresh fruit bother some people, caffeine, etc.) ....but remember foods will not cure the inflammation, but may help ease the pain a bit. Once you are in remission (with the meds that work for you) then you can bring in some of the foods with higher fiber. I hope this helps. Hang in there, this forum is very, very helpful with many young people with this disease.
Look up low reside diet under the www.mayoclinic that may give you great info.
Hope this helps.