spotting for a few hours and negative pregnancy test!!

Hey everyone,
So I'm supper worried. I didn't seem to get my period last month but I did have spotting for a few hours. I contacted a doctor and let them know and they said it could have been implantation or it's just the fact that I've been super stressed out and starting to work out can affect my menstrual cycle. I took 3 test at the beginning and one TWO weeks after the spotting since my director suggested it. Once again it came out negative. Should I still be worried?!?! Also a very close friend of mine had the exact same thing happen to her. We spotted on the same day for the same amount of time. Now we are both in the same boat. What do you all think?!