2008 Kia Rio Transmission problem

I had my car parked in a Walmart parking lot. As soon as I got back to my car, there was a huge puddle of liquid under my car. I brushed it off and went on my way thinking it might have just been raining or the pavement was previously wet. I drove to a second Walmart (about 7 km away). Upon turning into a parking spot, I overshot it and had to reverse to correct I backed up and then put my car back in drive. When I pressed the gas, the car jerked forward but didn't really move. I pressed the gas again and the car jerked forward just enough to position myself in the parking spot. When I was done in the store and I came back out, there was an even bigger puddle under my car and there was a trail of liquid that my car had leaked throughout the parking lot. I backed my car out of the parking spot just to see if my car was working fine, and I had no problems backing out. It wouldn't move forward at all. I was able to get my car out of the way so I wasn't blocking peoples and I had to get it towed across the street to the garage(unnamed big company garage). The garage claims my transmission is shot, but I have loads of peoples telling me I should have had problems with my car in the past few days/weeks, that a transmission doesn't just "go" within 5 seconds. The drive from the first Walmart to the second one and since forever, I noticed no problems with my car. It drove very smoothly. I haven't let the garage change my transmission, I want to get a second professional opinion before I dish out thousands of dollars for a new transmission.