My 21 months old had an ear infection 3 weeks ago, she had to get antibiotics by a drip for 5 days and she got better but her got swollen yesterday?

Unusual: Ear infection does not usually require IV antibiotic and thus the treatment was unusual. Although a drug allergy may develop up to 10 days after the last dose, the lapse too long to point to a reaction to the antibiotic. Since you didn't mention where the swelling occurs and how long it has been present, it is impossible to even give you a guess . If having breathing problem, rush to ER