hi there, first of all I would like to know whether you are a diabetic and whether you have hurt your self in the heel with a nail or thorn or any other sharp object which went unnoticed by you in the last few days? Your discription seems to fit that of cellulitis
of the foot and you will need to visit a surgeon and let him have a look at it as it may lead to serious consequences if not attended immediately. In the meanwhile you can keep your foot in an elevated position and can take an anti-inflammaatory tablet like brufen
to aleiviate the pain and redness
. Take care.