I've had headaches, sore throat, a cough, watery right eye and nausea. saw doctor on thursday, did covid test which was negative. he gave antibiotics and throat spray. this morning i have a rash on my stomach and back. i'm concerned it may be measles?

Based on limited information, this more likely a virus that would not benefit from an antibiotic. Taking the antibiotic for a virus causes a rash. Measles usually occurs with a high fever, spots in the throat (google search them), and red eyes. It is not common for people who are vaccinated, although the vaccine titers can lower with time. See a doctor a rash needs to be seen

Measles usually comes with a fever and abject misery. If you were immunized, it is unlikely you have measles - some other virus is more likely.

Another possible cause of rash may also be the side effect of the antibiotics, so definitely go see your doctor about this.