For several months now my period has started on the 14th of each month, last month me and my boyfriend had unprotected sex on the 22nd but since then have used condoms. I was due to come on my period a week ago and still haven't. I am experiencing some symptoms such as occasional nausea,cramping and sore breasts. I took two pregnancy tests (First Response) one on the 18th and one on the 20th both were negative, but is there a chance i could still be pregnant and its not showing up on the test?