But the birth control pill is most effective contraceptive among all the kinds. I don't think the pill will mess up your period. In fact, the pill is one way to regulate the period. I was adviced to take the pill because I had very irregular period. Ever since I started the pills, I get my period every 28 days. 

BTW, I was just as skinny as before the pill (of course not now being 5 month preg)