Rjesayr,
Most small superficial masses such as these are epidermal inclusion cysts or sebaceous cysts. These are benign and usually go away on their own. However, they can develop an infection around them. This could be causing the swelling and discomfort.
However, there are other things that can cause masses, of course. Usually, malignant tumors do not arise quickly. But, anything is possible.
If the swelling continues to spread, you develop other symptoms, or the area just doesn't get any better, you might want to see a physician for an evaluation. Over the internet, it is not really possible to provide an exact cause.
So, again, you may need a physician to examine the back and possible do some studies (ultrasound, MRI, blood tests), if this does not clear up shortly.
Good luck.