Birth Control Confusion ?

I started birth control three weeks ago and planned with my doctor to have my period every three months, so I would only take three rows of the pills and skip the fourth to avoid having my period. well I just finished my third week two days ago and I've been having light spotting and cramps like my body is trying to have my period? is this normal or what? it's quite annoying actually and I have no clue what's going on, help anybody?