I had sex on December 2nd. was the first time in about 6 months. only had it the once. 2-3 weeks later not sure which I had a period that lasted about 5 days I would say. normal bleeding. well I got my period yesterday it was normal for me type of bleeding. last night I got up several times to change my tampon and it's like my period came and gone in a matter of 12 hours. still today no period pad has been dry. so I had sex in December for my period in January now February and it lasted one day. question is what could of causes it. eating pattern has not changed. weight has stayed the same. not stressed, only 25 years old and I sleep about the same as always. as much as any one with a 8 months old who works 12 hour over night shift 36-48 hours a week. please any insite would be so appreciated.