She could have lice or mites. You can visually see lice, but sometimes they are hard to spot. Their eggs (nits) aren't hard to spot though. Does her fur look like it has a little glitter sheen to it? The nits are usually on the end of hair shafts, they look like little white dots on the end of the fur. If its mites, they are microscopic so you can't see them visually. Skin tests done by vets are usually inconclusive or you get false negatives. Wouldn't hurt to try her on Revolution, but I am not sure if that is safe for hammies? You can buy it online at
www.petshed.com. One drop behind the shoulder blades is all she would need.
Although, if she is rubbing against the side of the cage, she could be marking? Or is she scratching everywhere else too?