Hydrangeas can cause skin problems on some people. Certainly, if the skin rash and cellulitis occurred exactly where your skin was ripped, then it does seem likely that hydrangea was the cause. As I said in my article, the poisonous parts are leaves, buds, flowers, and bark.The poisonous component is Hydragin.
According to Healthline : "Cellulitis occurs when certain types of bacteria enter through a cut or crack in the skin. Cellulitis is commonly caused by Staphylococcus and Streptococcus bacteria."
I won't go into more detail here, but the upshot is that you should see your doctor as cellulitis can be serious.