The only time I commuted with a backpack, I hurt my back. I am also 5' 1" and the messenger bag was HUGE on me. I commuted using my Camelbak, with the bladder taken out; it was just enough room for some papers, make up, and my lunch. I would bring my clothes to work on the days I didn't ride.
My "commuter" bike is so heavy compared to my road bike, I chose to ride the road bike, instead of the bike with a pannier. I could never seriously ride with a backpack or messenger bag; I suspect there's a big need out there for someone to design a messenger bag that would fit short women. A WSD messenger bag!