Last time i had sex was 15 weeks ago. (mid july) had my period in july, august and september, but now it's 14 days late. took two pregnancy tests, but both are negative. i read about the hook effect.. is it possible that i am still pregnant?

Don't think so: A woman cannot get her regular period and be pregnant at the same time. However, she can get pregnant a week or two after her regular period. If she has some spotting or light bleeding that doesn't look like her normal regular period, she can check a urine pregnancy test about 3 weeks after sex (3 weeks after when conception might have happened) to see if she is pregnant.