During my huge weight loss periods I experienced the same thing. I didn't appear to lose muscle mass, however I couldn't lift like I could before.
During high school and year one of college I was around 240. I stopped drinking pop, soda, what-have-you and instantly reduced my calorie intake by about 600-1200 Pepsi calories per day. In two months I had dropped down to 170. I spent the next years lifting and training lightly and never seemed to exceed 180. Then came alcohol. I hovered around 190-200 for a few years. About a year ago I took some serious measures and got back down to 175 for a few months after a shoulder injury. Around then... I started dating my wife about a year ago. In that time I've gotten back up to 200-210 and I have no time for a structured workout regime and I'm one of those casual eaters who doesn't notice that I just ate a candy bar or an entire pizza. Ever. I'm working on it, and I'm sure that if I am mindful of it I won't have the problem anymore. I'm also cutting out beer for a few months.