I Was Diagnosed With Cellulitis On My Right Forearm 2

I was diagnosed with cellulitis on my right forearm 2 1/2 days ago. I was given a shot of Rocephin and cephalexin to take orally. The sore seems to be reacting positively to the antibiotics, althought the area is still red and approximately the same size. The tenderness originally stopped a couple inches before my elbow, but has now encompassed my entire elbow and my triceps/biceps area up into my armpit is very tender and sore. Should I go to an ER and be seen again or is this normal reaction to the antibiotics etc?