No period after one depo shot!!

Patient: I gave birth in November 2010. In Feb 2011 i had a depo shot for birth control.Before that shot i had normal period every month. After that shot, i had spotting EVERYDAY for a month!!After March, i have not had any periods till now (nov). I got off depo as i was gaining alot of weight and had headache at times. I have tested for pregnancy and all came back neg. Is its normal for me to miss my period?Did that shot have an effect on my body?If it does…why are doctors letting patients use them??