On and off bleeding for 2 weeks. 6w 6d pregnant

I’m 6w 6d and I’ve been on and off spotting/bleeding since 4w 6d. Sometimes it’s just pink and sometimes it a pinkish red. And sometimes it’s a brownish red. Haven’t had any clots with it. It’s there for maybe 5 min and it goes away. And sometimes I’ll go all day without spotting/bleeding once and then the next day I’ll have some. It’s only a mild amount. But the dr has no idea why I’m spotting/bleeding. At my 6w 1d ultrasound they didn’t see any blood in my uterus. So what is causing this. I have a left pelvic kidney as well so idk if that has anything to do with the spotting/bleeding. Has anyone else had this happen?