Not sure what’s going on

Have any of you or do any of you start bleeding between periods? I ended my period a bit over a week ago and started bleeding again a few days ago. This isn’t the first time it has happened. It stopped for a few months and I was having normal periods, but it started again a couple months ago. Do you guys know why that might be? I went to get it checked 2 years ago and everything was normal. Doctors even did an ultrasound and they found nothing. pregnancy has never been the cause either and I’m not pregnant now. So basically they didn’t know what was causing my spotting. Any help and suggestions will help!