I recently came into possession of an adorable Shorkie Tzu - a Shih Tzu-Yorkie mix. She has rather long hair, though, and there are tufts (like little balls) of fur behind her ears.
Why do these form, and how do I get rid of them?
Most likely these are just hair mats. They can be caused by scratching in that area, and can be aggravated if the dog's hair is getting wet when she goes outside. Once they've started, they tend to grow rapidly, so you need to take care of them or you'll end up with a short-haired dog!
You can break them apart by pulling on either side of them. Do it so that you're not pulling out from the dog's skin, which she won't like. If you pull parallel to the dog's skin they'll barely feel it. Repeat with the pieces of the mat until they're all fairly small.
Once they're broken up a bit, use a slicker brush to take out the bigger lumps, then comb through the area to take out the rest of the tangles. Make sure you get right down to the skin as the tangles can start to form there (and are even worse because a tangle there will pull on the skin, which could make the dog scratch more).
You'll need to brush the dog regularly to keep them from forming again. It's easier if you do it when her coat is dry. Always brush her coat before bathing her, otherwise you'll make any tangles worse. Using a conditioner while bathing her may help -- I've found it effective on some dogs, but useless on others.