Breast milk to formula

My 10 month old son has been exclusively on breast milk and now that my supply is gone I have started him on formula. I mixed breast milk and formula for about two months to get him use to it. Over this past week he has been exclusively on formula and today I noticed a diaper rash. Is this the cause? He is a frequent pooper so I am constantly change soiled diapers and putting cream on him. Any suggestions on formula? Or other suggestions to help?

Currently using pampers pure diapers, aquaphor diaper cream and gerber gentle start formula. I have found the combination of diapers and cream to be perfect for him