Why am I afraid of excercise?

I’ve always been really skinny since I was a little kid on through my teenage years but since I’ve turned 20 or so I’ve noticed an increase in my apetite and a decrease of my metabolism. I’m not as finicky about foods as I was when I was younger and I’m gaining weight. Now, I’m not overweight (or at least I don’t think so), I’m about 145lbs and I’m 5’1”. My arms are getting flabby though and so are my butt and thighs but my main concern is my tummy. It used to be nice and flat and now its not. My mom says I just need to work out a few days a week while I’m still young and I will lose it easily. But the thing is, I don’t really like excercise. Its almost like I’m afraid of it or afraid to let other people know I excercise. What is wrong with me? Does anyone else feel this way? Like they’re embarrassed about excercise?