 [ Symptoms ] could i feel implantation 2 days after unprotected sex if we did this slightly post forecast fertile days  i thought my chance was out the window according to when i was most fertile but my husband and i decided to start trying anyway we had unprotected sex on may 19 2015 and then some dull throbbing on my right side uterus and terrible lower back pain on same side starting last night may 20th and continuing into today may 21st my fertile calendar said i was most fertile from may 11 through 16 with peak at 14th but my cycles are about 31 days vs 28 [ Disease ] if you are being treated for sinusitis with a z pack keep in mind that a vast percentage of those cases are viral viral infections do not respond to any antibiotics so perhaps this is what you have my advice is for you to consult the medical provider who is currently treating you for patient specific advice in this matter