Okay, I've tried a search for this and I can't find anything that seems the same. If there is, my apologies for being such a fool.
After wearing some trail shoes in the ice for a few sessions, which are quite tight, and with a rigid heel counter.
As a result of this idiocy I've developed a prominent lump on the back of my heel, slightly towards the outside of each foot. Googling this suggests "retrocalcaneal bursitis". The only qualms I have with this sketchy self diagnosis are that (a) they don't hurt much and (b) I don't know how to treat them.
Anybody experienced anything similar or know the best course of action?!