How long after taking an antibiotic pill would side effects occur, I get nausea/headache hours after taking macrodantin (nitrofurantoin)?

Medication SEs. Not sure why you asked and answered your own question. It seems pretty straight forward and you have answered it. Am i missing something here? Do you have another question that you really want answered?
Antibiotic problems. You can have side effects within minutes up to weeks later. Your symptoms are probably from the medication. Talk to your doctor.