3 Days Before Period My Husband And I Had Sex 3 Days Before My Period. Could I Still Get Pregnant?

My husband and I had sex 3 days before my period. Could I still get pregnant? - 3 days before period

My husband and I have sex 20th August, 3 days before my time was. 3 days later it was 23 August no sign of my period. (I have a cycle of 28 days) was at the time. Am I pregnant? I feel bloated, my urine is dark yellow, I will eat, sleep, take vaginal discharge with pink spots, cramps, nausea or chest pain, I have no drugs, exercise or over / under weight. I had a hard day on the day before my time, but it would not be 15 days. (From today) When I went to the doctor yesterday and tested with negative results. Why is my period 15 days later I was pregnant and 3 days before my period? My level of hCG far too low?