Boils, lumps etc on one side of the groin can quite often be a result of one leg slightly shorter than the other, the boil will appear on the shorter side. A small shim under the cleat will fix this. Ideally you should get measured and a good bike fitter will advise the right depth of shim. Otherwise start with the thinnest under the cleat and go from there.
I suffered from this issue for years until a bike fitter measured me and shimmed one cleat. The issue has never returned.