no clue as to why? visiting a foreign country (virus, uncooked food - worms, etc)?
if you honestly don't know, and the doctors can't figure it out, i'd stick to the basics. eat something small all the time - every hour if possible. do compound movements in the gym - especially squats, deadlifts and bench.
though if you lost it that fast - 6 pounds per month, i'm guessing that something is going on here that is affecting your health. think again, as to where you were 6-8 months ago, and what happened that would cause this drastic of a change. stress could do that, but you'd have to be under a lot of it to loose that much weight.