having period much earlier than usual

hi... my last period was on 28 Mar, next period will be on 24 Apr (28 days cycle) but period came on 13 Apr, just 17 days after my previous period. I usually have normal cycle. This is my first time having much earlier period. went to doctor and he did scan and gave me medicine to stop period and did papsmear. no detail explaination and I'm still worried. any mommy experience having period much earlier than usual? thanx