2003 GMC Sierra GMC Sierra stalls

I have a 2003 GMC sierra 4.8L. 4x4 Auto. Yesterday my girlfriend took it to the store and when she came back she left the lights on and ran the battery dead. I jump started it and all was fine. The next morning I started it to go to work and when I took it out of park the idle dropped to about 300 RPM and the came back to about 700 RPM. And on the way to work every time I would stop at a light or stop sign it would do the same thing and it would stumble a little. Then when I left work it stalled when I put it in gear and would stall every time I came to a stop but otherwise it ran fine. I also noticed that it does not matter if I put it in gear or neutral, as soon as I take it out of park the idle drops way low. I don't have a check engine light on or anything either. I've never had any problems with this truck untill after she ran the battery dead. I don't know if the two are related or just a coincedence.