How much Benadryl can I give my puppy?

My dog Roxy is a bulldog mixed with catahoula, and she's about 4 1/2 months. This morning I noticed her upper left lip/cheek was swollen, and it got better on its own. Later on that same day, her bottom right lip was swollen also. I want to give her some Benadryl, but I don't know how much.