Welcome to NGA, mpoznanski. Without pictures, I would assume it needs more water. This time of the year, mopheads start to loose leaf moisture faster than the roots can replace it. Temps above 85F or windy conditions cause them heat stress. After a while, the leaves brown out from the edges inwards. Make sue that it had 2-4" of mulch and ramp up higher watering levels when temps are above 85F. The hydrangea should be in morning sun only (until 10-11 am) or in dappled sun.