What Causes Bleeding In Nose After Using Antibiotics For Sinus Infection?

5 days ago my daughter had a low grade fever which lasted 2 days then a cough and stuffy nose. Her cough has not let her sleep. she also experience diarrehea. I took her to the doctor today and she was diagnosed with a sinus infection and put on an antibiotic Cefdinir and Robitussin cough. She was given one dose of the antibiotic today at 11:30am. It is now 11:50pm and she just experienced a bloody nose. Should I be concerned?