Could be some sort of bites from mites in the straw, but could also be Staph aureus infection. Spots on the udder are quite common, not sure how it gets transmitted, if it is possible through bedding or not. Was the bedding a bit damp? It's not ticks is it? I had one spring a tick infestation in my tups as the broke into an old stable/shed in their paddock for some cool and shade and it had ancient hay/straw on the floor. They had a tick infestation on their bellies. Was treated with the usual stuff sheep get on their backs for flystrike, like Clik.
I have one that has spots on her udder every so often, I use the cream from
www.botanica.ie for her as an udder cream and it seems to keep it under control. Quite pricey though (but smells divine and I use it on my hands too)
Short of mucking out again I wouldn't know what to suggest...