If you don't really want to go to the doctor, go to a pharmacy store and buy a pregnancy test device. You've missed three periods .. you might be pregnant but hey, you need to confirm and see it for yourself :)
you should see a doctor asap if not go to the abortion office, pregnancy test are free, find out then do whatever you have to. the choice is yours the longer you wait the worse its gonna be and next time prevent pregnancy from becoming a terrible side affect!
I know how tricky Mother Nature can be, believe me, but this is the day and age to easily get yourself some birth-control. Pills, shots, IUD's...every child should be a wanted child and unless you are around babies and already know this, they are absolutely a bitch of a responsibility.