Help! Random spotting

Hi - hope im posting this in the right section. I got mild cramps this afternoon and thought it was due to gas/constipation (that happens to me sometimes) I took an imodium and after a while .. about 2 hours .. I was fine. According to Glow I'm in my ovulation window and we had sex last night. Peak day is supposed be Tuesday. TMI-- I went to go pee just now and see some blood on my underwear and leaked through to my pjs. Any idea why? It wasn't period blood.. it was like .. watered down kinda blood? I'm not sure how else to describe it. And sorta brownish. Anyone have any idea what could cause this? This is the first time this has ever happened to me ? I'm 33 btw.