What Causes Light Spotting Between Menstruation?

ok so i had my period on the 17th of last month it lasted 3 or 4 days then on the 20th and everyday since then i have had sex and the sperm has gotten inside me then on the 28th of last month i had another period that lasted 3 or 4 days and tonight i started to bled again but it is lighter than a regular period! could this mean i am pregnant? or does the extra bleeding mean i cant get pregnant?