Why am i spotting in between periods?

I have been spotting in between periods sometimes 2-3 days a week sometimes one week I spot n then another week I spot. What is the cause of it? Took test and it's negative everytime. That's all pregnancy test I take. The spotting is bright red no brown n pink. Couldn't be ovulation bleeding or anything. Have irregular periods n have a double uterus. Any help is welcomed thanks in advance.