Both sides of transmission? If you took the pan off to service the transmission filter, either you forgot to put the gasket back on (no insult intended, I've made the mistake myself), the pan bolts are WAY too loose, or you've seriously over-tightened the bolts and crushed the gasket, and now it will never seal.

Find out where the fluid is coming from and you'll know what to do then. The gasket bolts only need about 6 ft/lbs or so of torque to be tight. That's 6 pounds of force exerted 12" from the center of the bolt head - not very much at all. It's VERY easy to over-tighten these fasteners.