Hi Hooley,
The active ingredient in Benadryl is diphenhydramine and the advice for diabetics on Drugs.com is:
Diabetes patients - Diphenhydramine/Phenylephrine may affect your blood sugar. Check blood sugar levels closely. Ask your doctor before you change the dose of your diabetes medicine.
Diphenhydramine can have the effect of lowering blood sugar levels, sometimes to hypo levels, and it is recommended the diabetics should never take it without checking with their doctor. You don't say if the reading of 8.1 is unusually high or low.. If it was high it would probably be due to the prickly heat, if it was lower than you expected then probably due to the Benadryl.