ANY ADVICE...STILL GETTING A BFN..!!....?

I'm TTC #1 and did the whole OPK and did the deed after I got the positive...I usually have a 28 day cycle and was supposed to start AF on dec 20th, here it is the 23rd and still no AF, but still getting a BFN..these at HPT are getting expensive...i tested this morning and of course got a BFN...i'm having a little bit of AF cramps and my have been swollen and tender for the past week and a half, which is not normal...they usually hurt like 2 days b4 AF...anyways, when do y'all think I should re-test again and has anyone got BFN after u missed ur period and then finally got a BFP...thnx for the advice...