Trac Off light in a 2001 Cavaleir

I have a 2001 Cavaleir. The Trac light has only come on three times. The first time it came on I was going up a hill slow and it came on
then once I got to the top of the hill the car shut off. The second time it came on I was at a stop light and then when I stepped on the gas pedal the car was real jerky. I pulled over and turned my car off and turned it back on, the light was off but when I put it in Drive (i have a automatic) it made a dull thud sound. The car drove ok after the the thud sound but Im just really worried. If anyone could help
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