I'm Wasting Away!

Okay, so I'm not really wasting away, but it sure felt like it last week. I was sick for 3 days straight and hardly ate anything at all. No calorie intake=weight loss... it's a simple equation but not ideal for me. Most people enjoy the benefit of losing a pound or more after being ill, but let me let you in on a little secret-- I can't afford to lose any weight, even a single pound! Wanna know why that is? Well, it's because, naturally (and at a healthy weight) I weigh about 95 pounds. I know, I know, I'm a shrimp! My sister gets jealous about my weight and I envy her for the 6 inches of height she has on me, so there you go.
But losing weight is scary for me, as strange as that sounds. I stay at the same weight all the time because I expel as many calories as I consume on a day-to-day basis so my weight never fluctuates. So when I lose any weight, I have the absolute hardest time gaining it back because I don't overeat-- it's just not part of the way I eat-- I don't snack, and I stop eating when I'm full as opposed to when my plate is clean (strangely enough, that is a concept a lot of people don't get or understand).
So, all that to say, when I lose weight I end up worrying about how I will gain that weight back, instead of the common fear of keeping the weight off. I already lost 5 pounds last year when, stupidly, I did a cleanse with our friends that made me lose 7 pounds in 6 days-- not healthy at all! (everyone else did it for a full 10 but I couldn't afford to!) I gained 2 pounds back (just because I was putting food in my system) but have yet to recover the other 5. And then I was sick last week and lost another 2 or 3. I think I gained those back, but our scale isn't very reliable so I'm not 100% sure.
I'm not anorexic by any means-- I LOVE to eat, I just don't overeat. I eat 3 meals a day, but my stomach gets full faster than most. I've been trying to eat junk food/fatty food but it ends up grossing me out and so the whole purpose of stuffing my face to gain weight is lost. I'm not unhealthy or underweight (I don't think a doctor would be concerned, since I am 4' 11 1/2" and just all-around small-statured) even though if I had those 5 pounds back, I'd probably feel (mentally) better.
Oh, and exercising just freaks me out. I know I could gain muscle weight, but even the likelihood that exercising would make me lose even an ounce of weight terrifies me! So I'm at a loss as to what to do. I could snack more (on healthy things, of course) and try to gain weight that way, but snacking fills my stomach and so I'm not as hungry when it's mealtime... vicious circle! So I guess I'm just on here to vent about my "opposite weight problem" and see if anyone has any ideas for me or a reassurance that I would gain weight if I exercised... anyone?