Hamsters fighting. One is very VERY afraid of the other?

So today I bought two Dwarf hamsters from the pet store. They were in the same cage with a bunch of other hamsters. One is very little, and the other looks like it's almost an adult. I've been home for hardly two hours and they've been fighting every now and then. It's almost like the big one is picking on the little one, and now every time the big one comes near her she freaks out and flings her paws back. Every now and then they'll sleep together just fine, but I don't know what's up. They're both females. I want to say the older one wants to play (she's really energetic), but the younger one may be intimidated by her. Or maybe it's a dominance issue? Not quite sure. Help?