My period is late, is that because I have just recovered from glandular fever, or something else (see below)?

I am worried because I had sex two weeks ago, even though I used protection, my period is late and I am worried as to whether it's because I am pregnant, or if it's because I've just had glandular fever. I am experiencing mild pain, nothing too bad, in my stomach and on my bottom left side, didn't know if that related to it in any way. Please help!