yes you could very well be pregnant, you need to test, why didnt you think to test sooner? 4 monthes is a long time to go without a period, if you werent/arent pregnant you should go to a gynocologist and see why you havent gotten a period you could have something wrong with your body!
it COULD be the pill messing with you but it COULD be your pregnant, you CAN get pregnant using withdrawl i think its something like a 25% fail rate, meaning out of 100 women 25 will get pregnant using that type of "birthcontrol" (i use quotes because its not really birthcontrol)
also you may feel more at home in the teen pregnancy forum, and this post probably would have been better suited there or the am i preg forum (we still have that one right (im too lazy to check lol) people keep posting am i preg threads all over teen preg)
like shanyan said its probably too late for an abortion if you would want to go that route, but its never too late for adoption, adoption is NOT a bad thing, you can even have an open adoption where you get calls and letters and maybe even visits or can go to visit your kid
good luck
~alicia~