My Son Has A Large Bump On His Forearm. Looks

my son has a large bump on his forearm. Looks like a bite of some kind. Was in the woods and cabin this weekend. Swelling is red and raised almost a quarter inch and nearly 2.5 - 3 inches in diameter. Noticed this 24 hours ago. Gave him Benadryl and put cortisone on it . What do I need to watch for. Smallbroken bump with clear liquid in center.