baby's first diaper rash

we are going on month 5 and we have had our first encounter with diaper rash. but it's getting awful. she cries so much during the diaper change and I've looked up home remedies. so first question - how long after changing brands of wipes or diapers would baby develop a rash? and second - is it possible that putting extra virgin olive oil is making it worse or hurting her? She seems to get more upset when I'm changing her diaper. not sure what it's from but starting my elimination process. -- and before you go and say coconut oil, I'd love to get it it's just we don't have the money right now. and olive oil was in the pantry.