Please help..

I’m currently 22 weeks pregnant. At 3pm I started feeling mild period cramping that turned much more intense as time went by. I thought it was maybe constipation but it feels more like I’m about to have a heavy period without the blood. My lower back is also achy. I’ve been drinking lots of water and felt a little better after laying down for the last hour but it’s still there a little. Whenever I stand up and walk around it makes it worse. My doctors office closed at 4 and I have no idea what the on call number is if there even is one. Should I be worried about this and go to the ER? Or should I wait until tomorrow when I can call my doctor?