Blood after sex AGAIN!

last night for the second time I've had blood after we finished having sex. This time wasn't as bad as the last time but I'm concerned....it doesn't seem to happen EVERYTIME we have sex, but it's happened about every other time twice now....what could this be? The day after it happened the first time I was in alot of pain, so I thought I had a cyst rupture..now I'm not sure. No pain this time around. I am about 4 dpo. The first time it happened I was around peak fertility.