What Causes Spotting With Dark Clots After Having Intercourse?

For the pass week I ve been getting spotting. I did have sex 3 times since my last period which was last month on the 21st and we did use a condom and he did pull out and he ejaculated on my butt. I m pretty sure the condom didn t break and that none of the sperm came near my vagina when he came on my butt. Now I ve been having spotting for the past week and should be getting my period around this time. Well last night I went to the bathroom and wiped and saw that the spotting picked up a little bit and was a red color so I put on a tampon and woke up this morning to half the tampon being soaked and back to spotting again. Now I m having brown spotting and I ve seen 2 small blood clots. I ve taken a pregnancy test and both came back negative. I m on no type of medication or birth control and I ve never had this happen to me before. All the constant spotting is really getting aggravating. What could be the delay of my period?