What Causes Abdominal Cramps And Bleeding After Urinating?

i always have a late mentruation and last month my mentruation last for atleast 15 days. After that i notice that after im done peeing when i wipe it with tissue there's a claudy little blood on it. Its almost a month now and i wonder why is it like that.... and sometime not all the time a got this little cramp... i dont think im pregnant because i test myself and it said negative.