TMI HELP

Last night me and my boyfriend were having sex (vaginal) and I kept feeling pressure in my butt like I had to go poo or something. Well after we were done I was wiping myself with a rag and I didn’t see anything suspicious. After we went out on a date. I went to the bathroom and I noticed a brown like substance coming out of me. It didn’t totally look like poo but it did at the same time. It was like dischargy and brown. So I go to wipe my butt just in case and there was the same substance. Please tell me what happened! Why didn’t I see it when I wiped myself first?