What causes these kinds of bites?

I keep finding these very tiny red spots on my arms. They are way smaller than the random birthmarks and freckles on my arms. When I scratch the marks they go away for about 30 seconds before reappearing slowly. I think it's blood, not a mark. There are about one or two bites per square inch of my arms. We have no pets, my scalp isn't itchy, and I don't go out in the woods. What is biting me? How can I get rid of them?