1999 Saturn SC2 Transmission Leak after fluid change

I'm a bit new to this so I will try the best I can to discribe the problem. Two days ago, my car's transmission was due for a fluid change. NOT a flush. So I changed the fluid, which was dark red and had a gritty feel to it. I replaced it with Valvoline max life ATF and a fluid filter from AutoZone. I did everything according to the Hayne's manual. I secured the filter just as directed. I lubed the gasket with new transmission fluid. I even touqued the drain plug! But about a day later, a small puddle formed under the transmission filter. So I removed the air intake up to the throttle body to get a better look at the filter. And I also supported the car on jackstands. I ran a clean paper towel around whare the transmission filter contacts the transmission. I did not see any transmission fluid on the towel. So I looked under the transmission and found no leaks! I let the car down off the jackstands and reassembled the air intake. Then I ran up to the store and back to warm the fluid up. Then I looked under the car ad saw dark fluid accumulating at the seam of the transmission at the bottom. Please, can anybody tell me what this could be? It's the first time it's ever happened.