What Causes Even Small Bruises On The Back?

I have what looks like even, small bruises on my back. They don't hurt me and they aren't bleeding, nor swollen. They are just there. I haven't fallen or hit anything recently. Its on only one side of my back, and they aren't risen. I have a faint one near my spine then in a slight slope there are more in a sets of two, traveling in a horizontal direction. I don't know what it is. I have a history of shingles but they don't hurt or itch.