What is this lump on my foot?

Yesterday i went to an amusement park and wore the wrong kind of shoes.. Walked around for hours, and by the time we left i could barely feel my feet. If i was on them i was in horrible pain, but if i sat down they started to go numb.. Today they are still very painful and sore, but i also noticed a large lump/knot of the inside of my left foot on the back part of the arch.. It is uncomfortable to rub or push on.. What is this? Should i worry? It is also red all around it..