Surprise positive? Accuracy?

Im about 2 weeks late on my period but haven't long come off contraceptive (3-4 months). So just assumed it was me being irregular due to that. I was at work today and said about feeling a bit run down, sore breast etc (again just thinking a period would come pretty soon). Colleague joked about pregnancy, so on the way home purchased a test expecting a negative..... its come back positive and 3+ weeks! The test was a clear blue, should I retest? Im not sure how accurate these things really are and don't want to jump to conclusions...