How tall are you? I wouldn't try to gain the weight back, I'd just try to find out what's wrong, stabilize and stop losing, you don't want to go back to having a 42 inch waist do you? Why would you want to do that? But if you are experiencing rapid unexplained weight loss, and lethargy, definitely go see a doctor about it, and get yourself tested, diagnosed and have the problem addressed.
Coincidentally I just dropped 40 lbs myself, but intentionally, I started at 230 and am now at 190 too, I went from a 36 inch waist to a comfortable/loose 34 inch waist (I'm 6' 1"), I like being leaner, I look better and I'm more energetic.
Side note to those of you interested in losing weight, try the Paleo diet, that's the one I've been on, it's very effective and very healthy, google it, "The Paleo Diet".