What Causes Pricking Sensation Under The Armpit?

I've been woken by an extreme pricking sensation under my left armpit. I have had it before and thankfully I don't get it very often, but it seems to come and go for a short while and then disappear. It feels like and intense itch also, but under the skin, can you help? I am female, 38 yr old, height 5ft 6, weight 11st 3lbs. No medical probs, father was diabetic