What Causes Delay In Menstruation Other Than Pregnancy?

Hi, I hope you can help... I was on the pill for about 4 months and I came off the pill on the 8th Aug, I got a period on the 10th Aug for 3 to 5 days.... I then got my next period on the 12th Sep for 3 to 5 days.... I had unprotected sex on the 3rd Oct and to this day I haven't had a period. I think I'm about 10 days late. I have took a test and it was negative - could I be pregnant or is my cycle just messed up from the pill.