False negatives?blood test done 10 dpo

hi, please respond if you can because i am going crazy.  First of all i had unprotected sex the day i ovulated.15 days ago.  I went to the dr last week and had a blood test done 10 dpo.  Negative.  I took one this morning my period is due today but still nothing.  My test was neg.  I read in someone's post that antihistamines and diuretics can cause false negatives.  How common is this?  I take both claritin and drink tea and lemonade on a regular basis.  Is this likely to be causing my results to come out negative?  I have been cramping the last two weeks (not normal for me) and been tired and dizzy with some nausea.  Can someone point me in the right direction? Thnaks