NST Contractions?

So i went in for a routine NST today and I noticed the number on the contraction side moving around a lot. Anywhere from like 17 to 40's. I was wondering what number would indicate a contraction? I didn't feel anything that i thought would be a contraction, but i also have no idea what that would feel like. The nurse came in and said that it looked like i had a contraction and asked if i felt it. I didn't think i did, but now im not sure. Can contractions feel like the baby is pushing on the top of my uterus? I always thought it was the baby stretching my belly, but could it be a contraction? I had a doctor that i havent seen before and she didn't explain anything to me.