What in the world could have just bitten/stung me?

I just stepped out of my back door, I took a couple of steps and felt a sharp pain in the back of my ankle. I jumped and pulled up my pantleg to look, and there was a big, red welt there. Just then, I felt the same sharp pain in my thigh, slapped my leg, and felt something in my pantleg. I panicked and ripped off my pants and ran into the house.
Now I have two, burning welts on my leg each about the size of a nickel. If I didn’t know any better, I’d swear I was stung by a wasp… but it’s 1 in the morning and I didn’t hear or see a bee. What could have bitten me?