Weird spotting

So I ended my period on the 9th, and my fertile window just ended yesterday, but I had spotting last night and this morning. My boyfriend & had protected sex, then like 15 minutes later had unprotected. Each time he only did like 20 thrusts before he blew. He did not go inside me. We've done unprotected like 3 times & I'm not on birth control. But I also started taking antibiotics because I have a sinus infection... & I have inflammation in my rib cartilage. I have never spotted before.