Fuel Guage/Gas tank Issue

[YoMont]

Recently my 2001 Tahoe's fule gauge started acting eratic. The guage would just go from full to empty back and forth about every 30 seconds. Then it just stayed at empty event though I would fill my gas tank. Today I went to put gas into my tank and it took about $5.00 and then the gas started to pour out, as if it was clogged. I tried another gas station and the same thing happened. I am not sure what this is or why it is happening. Any advice will be helpful and appreciated. I will be taking my truck to the shop tomorrow but hope to get an idea of what it is before I get screwed by some jerk out for extra money