My 4 year old daughter was on omnicef for two days for an ear inf...

My 4 year old daughter was on omnicef for two days for an ear infection. She is now covered from head to toe in a nasty red rash. The rash on her cheeks looks as though the bumps have all meshed into one huge red bump. Her entire cheeks are bright red. The dr said it was an allergic reaction to the meds and changed her to Septra. I don't want to start her new meds until the rash is gone but it isn't showing signs of improvement as of day 3. I am wondering if anyone could tell me how long the rash lasts? Should I start her new meds for the ear infection anyway? And she is sooooo moody. Her meltdowns are on a nuclear level which is out of character.