What Are The Chances Of Pregnancy After Taking Ipills?

My period started 08/14/10. I had sex with my bf the night before. the following wkend i had sex again. I took plan b about 48-60 hours out. I know that plan b can affect your period however it is now 09/24 and no period. ive changed my diet and my exercise patterns within the last month. is it possible to be pregnant? I've taken three tests and they all say negative, but I have never been this late before. Would plan b change my period that drastically, or do you think the plan b plus the diet/exercise change/stress of maybe being pregnant is delaying my period?