Small lump in my ankle. Can someone tell me what it is?

I have a small lump in my ankle between my Achilles tendon and the bones in my ankle. It's not very big, but it's painful when I push on it and it bothers me when I run up hills. This just started yesterday and it's giving me a lot of grief. I'm planning on leaving at the end of this week to go march with a world class drum and bugle corps, but I am afraid this may force me to have to drop my spot. Can anyone tell me what it is and what I can do to treat it?