I'm Too Skinny! How do I bulk up?

I think you are right on track seeing a nutritionist or dietician at a hospital. That way they can help you knowing you have UC and some things do not agree with us at times. I would suggest meal supplements like Ensure or Boost in between if you can tolerate it with your UC. I tried with supplements but for me it made things worse because I tend to be very sensitive to certain things in food.
By the way, Welcome, Zach. Sorry that you have UC but you will find this a good board to be on and that you can ask anything about UC comfortably.

Your best bet is to find a high calorie food that doesn't bother you, then eat a lot of it. Based on your age and weight, I wouldn't worry TOO much about healthy-vs-unhealthy foods, just stay away from foods that upset your colon. If you want to just add muscle, make sure to lift weights. Either way, you have to eat enough to gain fat to gain muscle (working out just ensures that a greater % of the excess calories become muscle instead of fat).

hey, i used to weigh about 135 and now i'm 170. i can relate. in short-

1) protein smoothies (i use whey + yogurt + banana when i'm lifting) are your friend, and the yogurt (if it is live culture) is good for uc too!
2) work out with the big movements- squat, deadlift, bench, pull ups, etc. if you only do three things at the gym, squat, bench, and pullups!
3) to build muscle mass, you want to do less reps with more weight, to build tone, do more reps with less weight.

this is an oversimplification, but i don't want to write you a book. in short, eat more and pump iron if you want to be bigger, there is no easy way.

i have been a shrimp all my life and i did it believe me- YOU CAN DO IT!