I'm also a frog and fungi enthusiast, and that was my thought exactly.
The skin doesn't look irritated at all, so a tiny piece of mycelium-saturated debris must have just barely stuck on/in the skin and gave rise to a fruiting body. Or maybe the debris was embedded a while ago, the acute inflammatory response had long since passed, and as the frog's body pushed the encapsulated offender out the exposure to air triggered fruiting?
At least, this is the convoluted story I tell myself so I don't have nightmares about a frog-zombie apocalypse.