What if your boyfriend said this to you

Linds
My boyfriend and I been together for 4 years and we live together. I'm 24 and he's 25. Past 2 months i noticed I'm usually pursuing sex and not him. Now I gained about 20lbs in the last 4 months so I already feel like I'm not attractive. Anyways I asked him how come he's not pursuing sex and his response was "I don't like to have sex at night anymore And plus I'm getting older" 
To me this is an excuse. I don't know what to think. I'm pretty hurt because I don't know if he finds me unattractive or if there is someone else..: 
Help me ladies and I would like to hear your input.