Why is it that you shouldn't use allergy pills while also using medicated itch-cream?

What???!!!?? Who told you that? If you have a rash or bad itchiness, then a combination of a topical anti-itch and an oral antihistamine is fine.
"Overdosage" Some anti-itch creams are antihistamines and if you cover enough skin, they can be absorbed to some extent. This in combination with an oral antihistamine may cause excess drowsiness.