What Are The Effects Of Taking Birth Control Pills?

hi! i am an exchange student from europe. Icame to usa in August and was on birthcontrol. i got my period 1 time and then i stopped taking the pills cause i was scared of gaigning weight. i havent had my period in almost over 4 months but i want to take the pills again. can i just start taking them now? even though im not getting my period. there is no way i can be pregnant. if i was i would have been in 6th month now. so thats not possible. Can i start the pills now? is i bad for my body?