13 days late negative pregnancy test but symptoms of pregnancy?

my last period was march 2nd im 13 days late my cycle usually lasts between 24 - 28 days never later than 28 days, ive done numerous hpt and all negative ( used the cheap strip tests from ebay) ive had lower back pain for a couple of weeks my have become sore and swollen the last couple of days, some mild cramping (like period pains) we have been ttc for nearly a year now, could i still be pregnant is it possible as ive heard that some women dont get a bfp until there around 3-4 weeks late,