Missed Period, not pregnant (i believe)?

Hello,
I last had sex March 14. Used a condom and he didnt even finish inside me.
I had my normal period after this encounter from April 2-7. First question: its nearly impossible for me to be pregnant, right? Especially considering i did get my period after?
Anyways, I never had my late april/early may period as expected. It just didnt come. Skipped it completely which has never happened to me in my life. Im often late, but never skip one.
And now im 2 days late for this expected one, so im starting to worry a little.
Second question: what could be the cause of this?.. should i actually take a pregnancy test to rule that out even though it seems impossible? havent lost weight or anything, what other health issues could cause this?