Period twice in two weeks?

Usually I am extremely regular - virtually like clockwork every 28 days. In the past month I was 3 days late (took a pregnancy test that was negative) then got my period on Day 3. Lasted for 6 days...then spotted on and off for another days when another full period came out of nowhere yesterday. Aside from being embarrassed, I got freaked out and went to the hospital thinking something was wrong - but they said they couldn't find anything.