I started what I thought was my period last monday and by the next day it had stopped I had sex that thurs. And started bleeding but it stopped after?

Pregnancy test. If you have had an abnormal menses you should consider a pregnancy test. If these symptoms continue you should contact your md. S blood pregnancy test, thyroid function and other tests may be performed.
Irregular periods? You should take a pregnancy test whenever you are sexually active and notice a significant change in your monthly periods. If they continue to be irregular, call your gynecologist.