*hugs* I still get sleepy when I nurse my DD now! Not like I used to, but it will get better. Totally a hormone thing...you just get so relaxed. Having 2 I actually looked forward to bfing in those early days because even when I couldn't get a nap in, feeling that surge at least helped me chill out a bit (goodness know I needed it!).
I found it helpful in those early days to drink some cold water while nursing...kept me awake enough. Also, you could try some pillows under your arms, helping you to keep the baby up with no chance to roll off of you.