What Are The Beginning Signs Of Cellulitis. My Husband Was

What are the beginning signs of cellulitis. My husband was in the hospital about 2 months ago. He spent 4 days there before the doctors opened his knee to get infection to drain. He was then put on antibiotics. Now he is starting to complain of odd feelings and sometimes sharp pains again in his left knee and down into his leg. We do not want this to get as bad as it was before. What can we do before calling doctor?