What Causes Delayed Period While UPT Is Negative?

Womans Cycle: Hello, My name is Brittany and i'm having an issue with my cycle. I am now 11 days late of my cycle i've taken to pregnacy test to be sure I am not pregnant, even though I am not fully sexually active at this time. My cycle is usally on time every month and this is the first month that it is late. Is there anything else I should do because at this point I am scared and frustrated?