Is it normal for bodies to change as you get older?

I'm around 5'10-5'11 and I've been wearing size 10 jeans since my freshman year of high school. When I entered college, I told myself that eating healthy and moving around is a top priority. Since I had to either ride my bike or walk to my classes around campus. In the past several months, I've had to go up to a size 12 in jeans.

I didn't get much sleep during finals because I was studying and throughout the day I wouldn't eat until my stomach growls after breakfast. This summer I've had to help renovate my parents' home and when we got through, we had to order food because it would be too late to cook anything. I always drink water and I drink one can of soda once a week (because what 19 y/o drinks Kool-Aid?). I've been wearing gym shorts because it's too hot to wear jeans where I live.

Today, I tried on a pair of blue jean shorts (size 10), they were tight but I forced myself to wear them but minutes later my stomach felt like it was being squeezed and I started to feel lightheaded so I changed into some gym shorts.

Basically, I've never liked how my body changed growing up and I don't like it now.