What Causes Cellulitis In The Lower Left Leg?

I have cellulitis in my lower left leg in which I had a knee replacement last October. I just began my antibiotic last night. Is it dangerous for me to fly from NY State to WA State tomorrow? I am 76 years old, 5'5'' tall, weigh 180 lbs. I have also had a hip replacement on the other side. I have RA and PMR.