My gums directly behind my back teeth on the bottom row are swollen. It's not painful but the swelling is annoying. What could this be?

Pericoronitis. Pericoronitis is the probable cause. Your gums are swollen because they are reacting to an underlying infection. The back molars are probably your wisdom teeth or the back walls of your second molar which can be con fluent with the front wall of your wisdom teeth. This area, although seemingly self cleanable is not! you should floss it on a daily basis or consult for extractions.
Swollen Gums. The gums behind the back teeth are somewhat redundant. You may have something trapped between the gum and your back teeth, such as food. Try gargling with warm salt water, better yet, get a waterpik / sonocare. Both of these pulse water between the teeth and gum. Not getting better, go see your dentist.
Gum swelling. The causes of posterior gum swelling are somewhat age-dependant. Infants--due to teething. Children due to emerging molars. Other folks: food trapped between teeth and gums causing pericoronitis. Of course, with poor dental hygiene or lots of cariers, dental infections can do it as well.