Spotting after three week pregnancy scare???

For the past three weeks I've been in the middle of a pregnancy scare. (TMI right quick) The night it happened my boyfriend came inside me (or precum) and then asked if i was on birth control, which i am not, and me and my boyfriend were gonna get plan b, but he stopped speaking to me for a couple days so i never got it. Now according to my period tracker that was the last day of my fertile window and according to sites it's still possible. Now three weeks later, my period is 4 days late and I'm spotting very lightly and there are clots ( usually my period is somewhat heavy the first couple days) It's so light though. Like (excuse the tmi) I'm wearing a pad, but it's completely clean, it's only when i wipe that i notice the spotting. Now naturally i don't worry, so I'm not saying I'm pregnant...but i do need some sort of explanation.