Boyfriend acting weird now I've gained a bit of weight?

When we moved in together I was very skinny and I have put on some weight I have noticeable belly and I like wearing crop tops. My boyfriend always used to hold my waist but he rarely touches my belly area, and I like when he does. But when he does he always seems nervous and has asked if I mind before. To which I said no but he carry's on, this is the only thing that has changed about him over the years, and he says he thinks I'm beautiful still. Why is he being like this.
Boyfriend acting weird now I've gained a bit of weight?
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