I have an upper respiratory infection that started off with allergies.i have a newborn.i'm on a zpack. when am i not contagious anymore?

Depends: It depends on how long you have been ill or whether the problem is bacterial or viral plus whether the antibiotic is the right one in a bacterial infection. In the case of a viral infection, one is likely not infectious to other after about 7 days. The common bacterial infections requires close contact to be transmitted and usually unlikely contagious after 3 days if on the right antibiotic.