I have noticed this with stings and bites, the first few days it's not so bad, but then the venom starts to spread and the itchy/sore parts get bigger and itchier.
One thing that helps me is hot cold hot cold.
I've been awakened at night by a week old insect bite/sting; i go into the bathroom and run cold water on it, then hot, then cold, then hot.

you get those nerves completely confused and you stop itching. it's magic.
and a little Benedryl or cortisone cream will help too.