Weight issues.

Now that I have that out of the way, I can't seem to keep my weight up to a healthy weight. Not by choice either. I'm at 111 and I'm 5'3". I can get up to 117 if I binge eat but somehow I always end up losing it within a few days. It's getting to a point where my SO is scared to touch me and my clothes aren't fitting. My bones are hurting from nutrients running through me instead of going to their proper place. I take a multi vitamen but nothing is helping. I'm going to start taking ensure shakes to help but I'm scared. Again please don't say this is a good problem because it's not. It hurts a lot and it's limited a lot of what I can do. Thanks for any comments and opinions.