Hi PineMum, yes I would say it is fairly normal - it happened to me too. I have a feeling Tracey says something about it in the first BW book (don't have it to hand, so can't find the relevant bit). Apparently something is secreted in your body while your breasts are being stimulated by the suckling which can make you drowsy.
Do you have the BW babies book? I'll try and find the right bit and PM it to you...