Uterus/Stomach gets hard ?

Diana • 20 y/o. Mommy to a beautiful baby boy named Jason, pregnant with baby #2 due Sep 2018.
So, my uterus will sometimes get hard and (this is gross) but then I'll feel like I have to fart. I don't feel pain but it's just uncomfortable. It's really weird and I don't know what's going on. I called my OB-gyn and she told me to lay on my left side and drink water. Has this happened to any of you guys? I'm 26 weeks.