It's clearly an infection. It would help if you already followed the advices of the prior posters.Hope you were already given anti-tetanus shot for that.
Sounds like there is still some rubbish left in the foot and like Fruittibiscotti says it may also be infected.
The problem with it being under your foot, is that every time you put your foot down, you are applying pressure to the sore. Use the cushioning pads that are available to treat corns to help keep pressure off the spot. You can purchase these from the large supermarkets and also from the pharmacy. If the skin is not broken, you can apply a paste called magnesium sulphate (you can buy this from the pharmacy). This is a drawing ointment that will help to draw out any rubbish to the surface. Follow the instructions on the container.
Soak your foot in a bowl of warm salty water, this will also help to clean the wound.
You should go and get it checked out by your doctor who may prescribe an antibiotic and refer you to the nurse or the ER to get the area cleaned out and check if there is still any bits of debris still in the flesh.
Possibly an infection. I'd go to a clinic to get it checked out and disinfected by medical personnel.