ooh. that makes sense thanks :]
Depends on where you're running. Outside? In the heat? My fingers and feet will swell up when I run in the heat. Exercise in general can cause some light swelling in the body. I'm not a doctor or an expert, but I'm guessing it's a healthy bodily response to exercise.
Here is some general info on why it happens. It's related to weight-lifting, but if you are working the muscles in your legs, it could also apply to running.
http://www.johnberardi.com/articles/training/weightlifting.htm