1993 Ford Aerostar transmission leak

I have a 93 ford aerostar with, 137,000 miles. The van ran exellent, but all of the sudden it felt like if you pressed the gas pedal and brake pedal at the same time! It lost power and when I coast, it sort of brake on its own, like the trani forced it to brake. Then, I got to 35/40 mph and the van started shaking. So I pulled over and when I cheked underneath with the van on, there was a big leak right at the end of the transmission and butt to the engine, right underneath. I turned it off. Then after 2 hours I put transmission oil, 3 quuarts, turned it on and checked and there was no leak! So I decided to drive it, and drove it for 15 min to my other destination and when I got there, I checked and NO LEAK! It worked fine, I ran it over 40 to hit 65 on the freeway and no problems. Whats the problem with that! I came from milwaukee to chicago, but I dont want to drive back if it would do it again, can you tell me the problem please. I really appriciated! Thanks!