FTM 21 weeks pregnant with weird symptoms

Hi guys! I woke up at 3:30 this morning to pain like cramping in my lower abdomen. It's not what my round ligament pain, it's more crampy. I went back to sleep and tried to sleep it off. Woke up 3 hours later and it was a little worse. I again tried to sleep it off. It's now 12ish and I woke up with a back ache along with the pain in my abdomen and I'm scared. Could something be wrong? Is it just stretching pain I haven't felt before? It's sitting more in the bottom left side of my pelvic area. I know a lot of you will say to call my OB, but I don't want to hassle her if I'm just going through stretching pain, which is why I haven't called her. Any advice or opinions are welcome because it's freaking me out.