Kate this is all really normal. It's cause the radiotherapy messes with your lymph system making you highly prone to cellulitis. Now the good news is that it is probably NOT NOT NOT cancer. BUT cellulitis is nothing to mess with. It can lay you up in the hospital in double quick time. Keep taking the antibiotics, and when you see the surgeon on monday ask if it might help to switch you to others. If you've got cellulitis badly sometimes IV antibiotics are the only way to deal with it. If, before monday you develop a fever (over 37.5), please go to A&E right away. Probably the antibiotics just need time to get working.
Anyway, try to stop worrying--isn't the waiting room awful? <<Big hug>>