96 Cavalier fuel pump/guage

guage registered full and I know it was just under 1/2. By end of day, guage registered correct. This morning I went out to go to work and it is full again. It would be nice if it really was Full with the gas prices but I know it is not. Was there something the mechanic might have done to throw off the gage or are they independent? Thanks in advance.

First I have to say I have no specific knowledge of your car, and what I am about to say is just from what I have picked up here and there but here goes....

The fuel pump is IN the tank (likely). The float gage that measures your tank fuel level and sends the signal out is ALSO in the tank. Considering that both are in the tank, and that a mechanic just was working in that part of the car just prior to your gauge starting to malfunction... yeah, I would say there's a real good chance he might have boogered something, knocked it or some such.