Since you've been eating about 1200 a day, I'd try eating 1300 a day this week. Then next week, or the week after, add another 100 calories. A word of advice: don't step on the scale while you're "upping." It usually causes some water retention as your body gets used to the food. Any weight you gain while upping, you'll lose (and more) once your body adjusts.
As for adding more calories, 100 calories is pretty easy. Do you make your oats with water for breakfast? Try using lite soymilk (50 calories a cup). Are the WW meals good for your tummy? I am skeptical of anything packaged. As for non-processed grains, I like to make my own breads and buy whole grain (not whole wheat, of course) pastas and bagels. You could try eating a little more protein with each meal or even have some fat-free Haagen-Dazs sorbet (a half cup is 120 calories) for dessert. I don't know how well you react to Peanut Butter, but you could eat 2tsp for about 65cal. If you'd like an idea of what I eat, I usually post in the daily "checking-in" threads.
all those years it wasn't IBS - it was celiac!
send me an email: firstname.lastname@example.org