What Causes Spotting And Clots Between Periods?

Hello. My husband and I may think I am pregnant, but I have taken several at home pregnancy tests and they all came up negative. I just recently had a period, but i have been having some bleeding or more like spotting issues for 5 days now, and after the time I have stopped my period. I have also found some clots in my underwear. I am worried, and I am not sure what it is, we think it could be implantation bleeding because we have been doing alot of research online. Is there any reason to believe I may be pregnant or is there something serious going on and I should go to the doctor about?