Ouch a blister!

My foot has been bothering me for a day or so but I didn't really think anything of it. I figured it was maybe from wearing my flip flops or something. Well just now I crossed my legs and noticed something on my foot. Yup...it's a large blister on the ball of my right foot. I don't know what it's from. I don't 'think' it's my new tennis shoes since I've had those for several months now. I haven't worn any new shoes though so it's not from that. The only thing I can think of is that it's the friction from the running. Boo....it hurts!