With my oldest I started having a period the month after delivery, while exclusively bf. With my second and third I got it at five and seven months, so yes, it can happen. My mother never missed a period while she nursed, yet a cousin, doesn't get hers until after she weans her children. Each nursing situation is unique so you can't really go by what happens with other woman.
I think everyone is unique with this..My son is 6 months old and I am still b/f and havn't started my period yet..with my daughter..I didn't start til she started baby foods and her nursing decreased..I really think it depends on your own body...and yes, it's completley normal..just the way your body is working for you...and remember..it's all hormone derived! Take care and hope this helps out!!