way too skinny female

I desperately need help gaining weight, quickly! I am a 5' 5'', 30 year old female. weighing only 87 pounds. No matter what I eat, I seem to be losing weight instead of gaining. I feel fine, full of energy, I don't get sick, eat a healthy 3,000 calorie diet, and partake in yoga and walking daily. My only problem is that I go through terrible boughts of diarreah (diarrhea), some days I go ten to fifteen times. I've had an endoscopy, thyroid panel, celiac test, and stool/blood tests, all of which were normal. I thought age would've kicked in by now, but so far it hasn't. I am so tired of looking like a cancer patient, and not being able to fit into any clothes. What gives? Hs anuyone ever heard of or encountered my problem? Please help!