WHY WOULD MY CAR LOSE POWER IN TRANSMISSION?

I'm assuming that you checked the level? and the color was red?...just checkin'

if the level was low, I'd start lookin' at the source of the leak, perhaps a loose fitting on the transmission or on your radiator (likely suspicion)-

LEVEL WAS HIGH THE COLOR WAS RED

uh oh, better get aamco....no seriously I'd go to car-part.com and fetchez-vous a replacement from a wreck with lo miles on it.

it was the torque converter

...well the torque converter has to have fluid...did you not fill, then run it throught the gears, stopping at each one for a while? The thing, if empty will take time to fill and work properly, in fact some even have a plug to let out/install fluid in this thing, although it's supposed to take on more fluid as part of it's circuitry will not "come alive" until it's full of fluid...so perhaps your assertatoin that "LEVEL WAS HIGH" could have been true at the time, the converter takes almost a gallon all by itself, and needs to have some fluid in it, if it's gonna work at all!