What Causes Pus Filled Sack In Belly Button?

I have something that just came up in my bellybutton. I looks like a water/pus filled sack. it is deep inside my bellybutton. the area around my belly button has been sore. Not painful. My dad thinks it is because i have started working out and have maby pulled a mussle. It really ist even sore around the belly button any more. but their is definately something different than i am use to seeing