I've seen that before on some dogs with sensitive skin and/or allergies
. I used to have a German Shepherd who got it all the time when running in the forest just from the testie's hanging so low (they were almost at his knees, not big but hanging low). He would scrape on things and have cuts and bruises on them all the time and then would lick and chew himself Summer or Winter. One Winter when he was about 5 years old it got so bad in the high snow that it was raw and bleeding
for a week and I decided to get them snipped off. :-)
Stiggy is so close to the ground, if he walks even through low weeds that irritate his belly and sensitive area will make him lick and chew himself. We keep the grass as short as we can, but it doesn't keep all the allergens away in the summer. There is a remedy called Aller-Gen available at the health food store, if it reocurrs it might be an idea to try. It should stop soon though as sadly, summer is coming to an end.