Rash on neck after starting azithromycin. I'm on day 3 of 5.

Maybe allergic. The timing is obviously consistent with an allergic drug rash. However, stopping azitrhomcyin might not do much good. Azithromycin is very long acting and will be in your system another week to 10 days, even if you stop taking it now. So finishing treatment might not make any difference. Contact your doctor and follow her or his advice.