Weird experience--Alpha particles?

I was outside of my house, nice day but no direct sunlight (I think it was behind the house at that point), not too hot, not too cold. Really a perfect day to lay back and enjoy the good weather, which is exactly what I was doing when I noticed little `pinprick' sensations on my skin.

It was the strangest thing, my first thought was that it was lightly raining, but there was no water to speak of on my skin, on the tabletop near me, etc. There were only thin and sporadic clouds in the sky, but they seemed to form a shape like iron filings on a magnet, a sort of cardioid pattern.

I walked under the patio, and the feeling stopped. I could stick my arm out from its cover, and feel the pricks only on that arm.

Anyone else have this?