Rashes can develop due to many reasons and might not be due to the antibiotic. If you develop a rash whilst taking antibiotics please discuss with the person who prescribed you them.
Please take note of where the rash has developed. Did it cover the whole of the body or was it restricted to a small patch? Was the rash painful or itchy? How long after taking the antibiotics did it start? Did you have other symptoms or feel unwell whilst you had the rash? If possible it can be useful to take a picture to show your doctor.
Antibiotic allergies can restrict the arsenal of antibiotics which can be used to treat infections and can sometimes mean that first-line treatment cannot be used. Discussing side effects or possible allergies with a healthcare professional can help to determine how likely the antibiotic was to have caused the rash and ensure safe antibiotic use in the future.