What Causes A Delayed Menstrual Period And Negative Pregnancy Test?

hey, I came of the pill about 4 months ago and hav ehad regular periods , however i am two weeks late. I dont use concertraption with my boyfriend as trying for a baby. however i have been took tests and the last one was monday and it was negative. could there be a chance im still pregnant? or could it be something else?should i retest?