What Causes Vaginal Spotting After Unprotected Intercourse?

Hi...im having blood on the tissue after urination.i had unprotected sex and contacted bv and was prescribed flagyl.i took it for 3days and notice that theres blood on the tissue after i urinate in the morning,i stopped the drug for a day to see if its a side effect but i still notice d blood on the tissue this morning.also having nausea with it.the blood isnt bright red or much,looks like its mixed with vaginal discharge.i woke up with upper leftback pain also feeling like a cramp,i drank water and went to d toilet and felt better.pls im worried about the blood after urine.no significant pain,or frequent urination as such.