To answer your question:

Back Pain
Riding in the drops for long periods of time and not being accustomed to doing that on a road bike.
Riding a road bike for long periods of time in any position and not being accustomed to it.
Not professionally fitted (wrong frame size, stem size, spacers, saddle height, bar angle, etc.)

Ass Pain
Not enough miles on the saddle (many have to break in)
Not enough miles on your ass (if you're not experiencing numbness this is likely the cause)
Not professionally fitted (if you're not fitted properly you might have an improper amount of weight distributed on your ass)