What's wrong with my back?

Okay so for the past couple months I've been having this 'period pain' in my lower back on my left side. Usually its a dull aching feeling but sometimes it gets kinda bad. Not enough for a trip to the ER though. I feel it almost every day. It starts to happen when I start moving around or walk anywhere. Even if its just walking around the house. Also, idk if it has anything to do with this but no I'm not pregnant, I've checked multiple times. Any help is greatly appreciated, thanks!