Apart from getting a full check-up (as someone said, thyroid issues are so common nowadays and can lead to weight loss), I think it's more important than you might think to tell your boyfriend to not interfere like that. Gaining weight (or losing for that matter) is not about "accomplishing a goal" and struggling with it does not mean you are a failure. It's about finding a way that works and embark on the journey that can take many twists and turns. The less worried you are about "the goal" (or the number on the scale), the less anxiety-riddled you will be about this. You don't have to answer to him. If my boyfriend did this to me regarding weight loss, I would seriously consider leaving because I don't need anyone to pressure me about things that I pressure myself about enough. He needs to back off.