At first, I thought it might be a skin reaction to the clipper blades (my legs looked like that after I tried an epilator - ouch!). However, this theory doesn't really fit with your description of when these lumps appeared.
It could be a bacterial infection in the hair follicles. Maybe a mild allergic reaction to the lice treatment?
As for overheating, the weather is pretty confusing right now. Hard morning frosts, biting wind chill factor one minute then the wind drops and suddenly we have blazing sun! Even I was, er, "glowing" under my padded jacket this morning (naturally, ladies don't sweat . . ). Then back comes the wind. The temperature extremes from hour to hour must be a nightmare to rug for.
Sorry, I was no help at all Keeley. Looking forward to hearing what your vet makes of it next week.