Uterus Spasm

Michelle • 4 kids, #5 due 8-25-15
So, I've felt baby move already (I'm 17 weeks) but about 1 time per day I have what seems to be a muscle spasm on the left side of my mid belly. It feels like a two year old kicked me really hard from within, but without the pain. It creeps me out. It happens mostly when I'm turning to look at something or reaching for something (and tightening my stomach/uterus muscles). Has this happened to anyone before? I've had 4 children before but never felt anything like this before.