Why does my Rear bike tire slow down when Pedaling?
By far the most likely reason is that you've over tightened the bearings.

Did you feel as if you needed 3 hands to get it back together? If not, you did it wrong. :)

As usual Sheldon has a pretty good writeup on this, but the basics are easy enough.

Put one side back together, tighten it up as best you can. Set the cone on the other side to the correct tension. Tighten locknut on second side without turning either cone.

The tricky part is the last bit. You need to hold the cones on both sides at the same time whilst tightening the locknut and you really need 3 hands.

You will most likely have to try a few times to get the tension just right. When it's done it should turn freely but not have any wiggle.