Skinny after stopping medicine? Boost appetite?

I have always been skinny my life. When I was younger, I was severly underweight. Then as I grew, I moved to underweight and it seemed to get better and better then. This year, I was diagnosed with bipolar modod disorder and had to take medicine. As I had troubles getting sleep, I had to take one pill which helped in getting sleep and it's also said to boost my appetite. But a few months ago, I stopped that pill because I now have no troubles with sleeping. But evidently, my appetite dropped a lot, and now, I'm back to severly underweight. Is there anything I can do and not eat that pill? Although it's said to let you sleep well, the after effect is that you will feel lethargic when you wake up no matter how long you've slept.