Possible minor stress fracture (stress reaction?) although I think shin splints more likely.
I did a few races in the week before the OMM with very tight calves; insufficient warm up.didn't think much of it.then no warm up at all before the OMM itself, due to rain, cold, having the bag, etc.
Then running with the extra weight of the bag, especially downhill, put it over the edge I think.
Wisdom - warm up properly! And I've been doing a lot of stretches - calf and quad and shin, on the basis that they can't hurt.