Question: Problems with my DD brakes ?

[killain]

My DD is a four door 2002 Chevy Cavalier. About a month ago I got a thumping noise from my rear drum brakes. I took it in to my mechanic, and upon removing the brake drums, they were clearly so rusted and out of round that they clearly shot. But the rear brakes had good amount of lining on the shoes. So we installed a new set of drums, and I took off. But for the next day the brake drums ran HOT, Very HOT ! So I went back and this time we completely removed all of the rear brakes and replaced everything with new shoes, hold downs hardware, everything except for the brake shoe cylinders which were fine, no leaks or any damage at all.

We reassembled everything and off I went. Brakes were working perfectly, not running hot and the braking action was perfect. That was three weeks ago and today on the way home I stepped on the brake pedal lightly and again I got the thumping noise as I had in the begining? So now I'm really confused, the brakes were not running hot, the stopping action was perfect, but now clearly when I step on the brake pedal lightly and moving at around 10-15, 20 MPH I get the same thumping noise, just like when the brakes drums were severely out of round before. What can cause the drums to warp so badly in 3 weeks. And i stuck my finger into the holds in the wheel and touched the brake drum and it wasn't hot ? :W