the slightest spotting

Marisa
Hi ladies so I have irregular periods. About a week and a half ago to two weeks I thought I was starting my period but it turns out I would spot a little and then it would go away and then I would spot little bit heavier and then I would go away. So I thought if that was my. It only lasted like literally two days and is over with. another regular period in the books.  
On Wednesday I have sex and today while driving I began having cramps so when I got home and I went Pee I noticed three small somewhat dark blood stains.  I thought great my period is starting again another weird period This month. But after I wiped I looked at the toilet paper and there was no sign of blood no light pink no dark red no brown discharge nothing. And actually as I'm typing this I'm getting cramps again. Any idea what this could be? Has anyone else experienced this?