Should I call my OB?

For the last 2-3 weeks I've been feeling my baby kick pretty much all day everyday and during the night. But the past two days I've been getting some period like cramps, no tightening of the belly, and her movements have become a lot less. I haven't had any bleeding and I am 23 weeks along, but I am a FTM and I'm just a bit worried. Should I call my OB tomorrow? Or could this just be uterus stretching?