Do you know what type of trans it is ? Chances are it is a 4L60e. What is the Milage? There is a few things that could go wrong. VSS Sensor,or worse case bands and hard parts. You should check the fuild to see if it is burt.If the color is dark there are some issues there. If you want bring the car by the shop and ask for Jim (Master Tech from GM). No charge to check it out.
Bobby 813-961-0609