Car jerks

I have a 2003 chevrolet impala. When I press the gas pedal it acts like it does not want to shift. When it does I makes a hard jerk and almost lands me in the back seat. Please help? Any suggestions?

Have you checked the level of the transmission fluid? If not, do so and report back on the level, the color and the odor of the fluid.

When was the last time that the transmission fluid was changed?

I just had work done on it this weekend. I guess I need to see if the mechanic put transmission fluid back in. If he did what could be my problem? Thanks for your help!

Did this problem start after you got it back from the mechanic? What work was performed?