You didn't say if you are female or male or if you smoke cigarettes. However, regardless, being what is sometimes called ectomorphic will make a difference.

You did, however, say that you had been ill at one time because of being underweight, and you had worked to put on weight. That makes me wonder if cigarettes might be contributing to the situation. I had the same problem once, years ago, when I smoked cigarettes (cigarettes discontinued long since ) since the combination of cigarettes and being underweight in those years--and maybe adding in the additional factors of lack of sleep or adjusting to different climate or different time zone--can make a person more susceptible to whatever germs and viruses are floating around out there.

I do agree with the last poster, however, who indicated that it is probably water loss. At least a lot of it could be, though not necessarily all. In your case, maybe you just tend to shed pounds naturally anyway, and combined with water loss, you could easily be showing more loss than would other people. And, as many people will tell you, the "difficulty in gaining weight" is a problem that will likely go away in time. : )

ETA: OP, I have just read your other thread in this forum, about being ultra-susceptible to temperatures that wouldn't bother most people. So I have to ask: Have you seen a physician about this? It sounds unusual, if you don't mind my saying so, and maybe ought to be checked out. If you've always been this way, maybe just an unusual anomaly, but if it has developed gradually over the years, you might ought to ask about it. Hyperthyroidism a possibility? ? ? ?