Red itchy bumps on skin : insect bites or skin allergy ?

I have been getting what appears to be bug bites on my legs, ankles, stomach and back sporadically for a few months. They come out of nowhere, and they look pretty much like a mosquito bite and are extremely itchy. They appear red but when I scratch them they almost become invisible because they turn the shade on my skin. They seem to last for at least a week and cause alot of discomfort, then it just goes away. But it seems to happen every few weeks, I was wondering if you know what the problem could be?