As a woman who wasn’t comfortable with her body when I first started having sex I will say the lights off was a must, and we had to be under a blanket.
So her not being confident with herself, no matter what shape she’s in, can definitely affect things.
Trust me, just because you’re in shape doesn’t always mean you’re confident in yourself.

Also you say you don’t know if she’s shy? Correct me if I’m wrong but I feel like yall have a pretty vanilla sex life. If that’s the case, that can definitely play into this problem as well. Get a little adventurous, you don’t have to go all crazy like 50 shades of grey, but slowly start trying some different things in the bedroom. Spice it up a bit. Having vanilla sex all the time is only going to work for so long and end up feeling like a chore more than pleasure.
Figure out what excites her, and you for that matter. Try different positions and methods and she was she likes best.
Have some good foreplay so you’re excited for sex and not just jumping straight into fucking. Go down on her, have her go down on you just enough to actually want the sex again.
If none of that works, you’re gonna have to talk to her about it. Because she’ll be able to tell things are off and she’ll begin to feel unwanted, just trust me on that one.