What Causes Irregular And Delayed Periods?

hi i wantwd to no should iii go c a doctor because i had to periods in march one from the2-4 and the other 26-28 the first one wa light the other normal then went off this month im 3 days late...could i be pregnant...me and my boyfriend had sex saturday as well he pulled out but went back in raw 2nd time he did not cum but went to sleep inside of me.could his cum still be inside of me.