What Causes Spotting In Between My Periods?

I had my period on March 11..had unprotected sex on March 20. Got my period again April 11..lasted only 3 days (usually lasts 5 to 7 days) had sex on April 19th..bled a little bit right after sex..no blood the next day..now today the 21st I have cramps and bleeding again. More than spotting but not a regular period. I m not on any birth control or meds..no health problems..and I have never had mid cycle spotting. What could this be due to?