I agree I would go to the doctor for sure by now if ypu have not had a period in 12 weekz. If you are not pregnant you need to find out why. Here's hoping its because you are and all is well!
It is very rare to get a false positive so I would assume you are pregnant.... You might want to call your dr since it has been so long. Sometimes tests dont work so you can get a negative even though its positive... I would just set up an appt even if you really 12 weeks late something is going on and you should see your dr either way.