I had this the other day! Still do not really know, although guessing i am not pg I am going to test again next week, to be sure. I have heard that you cannot get a false positive though, and also that ClearBlue are not that sensitive, and to use First Response (which is not very helpul to you!) Can you test again tomorrow? Maybe it is a bit early.
id say the same as liahgen and try again with your morning wee. if you've been drinking alot to make you wee for the evening test your wee is going to be very dilute, especially if you arent very far along pg wise.
sage advice if I could just sleep. BFP is a bit of a disaster on all fronts but one we would / will end up loving I suspect. Two bed flat with mortgage DD1 2.8, DS1 16 weeks and a real need for two incomes. So probably best if BFN