What Causes Delayed Period And Cramps After Taking Pills?

Hello, I am very concerned because I have a very regular period was Aug 7 and ended on the 10th. I had protected sex and my next scheduled period was supposed to start on Sept 3rd and it did not come, in fact I had a discharge that was liquidy and clear. The next day I had unprotected sex twice and the discharge continued. On Sept 6th I took Plan B. Now that it is the 9th, my period is almost a full week late, and I am worried that I might be pregnant. I know that taking birth control can change your period, but my period was due before I had unprotected sex. Also I have been experiencing cramping as if I am about to start my period but it hasn't come yet. I have taken about three pregnancy tests and they have all come up negative. Help please!