This sounds like it could be bumblefoot, a bacterial infection of the heels.
The following articles provide more information with pictures:
Bumblefoot is most often seen in rats who live in wire cages (and don't have a solid floor to stand on), have dirty bedding or bedding that is rough and irritating (such as some wood beddings and corncob) or that are overweight. Other rats just seem to be genetically predisposed to having it. The most effective means of treatment is Blu-Kote or Blue Lotion, and changing the living conditions to prevent the problem from recurring, if possible.