blood clot after sex

After sex with my SO, I went to the bathroom and a bright red blood clot came out with a thin milky colored string. I’ve been on birth control for about 3 months. I’ve had really light, dark colored periods since, so I don’t think it’s that. My hip bones have been sore (which I attributed to sex positions) and I’ve been very nauseous (which I attributed to the birth control). I was tested for STDs last month during my annual pap and all test came back normal. Because of the symptoms,I took a pregnancy test last night which came back negative. Then during sex tonight I felt like vomiting I was so nauseous. After he came, I went to the bathroom and the clot came out. I’m wondering what it could be as this has never happened before. I take Lo loestrin Fe birth control and zoloft for depression. I also have genital herpes but haven’t had an outbreak in years. Could I have miscarried? Any ideas what else this could be?