I agree that you should go in for an ultrasound to help date the pregnancy. The period you had 12 weeks ago could have been implantation bleeding so you could actually be 3 months along or it is also possible that you could be pregnant with multiples. Good luck and keep us posted on how it goes.
Some people don't show positive as quickly as others, if you had a faint line and then waited 2 weeks and got a positive, you were probably pregnant atleast when you had the faint line. The only way to find out for sure is to go the the doctor and have them do an ultra sound to find out exactly how far along you are.
even if the line is faint its supposed to mean its positive. so when ever you took it and it was faint then you should have been pregnant then.