Red bumps appearing after mowing the lawn?

[mbaker1431]

I work for my uncle every week, mowing the field next to his house. I've been doing this for about two months and nothing strange has happened until just last week, when a small cluster of extremely small pinkish-red bumps appeared at the base of my thumb.
They don't itch, they aren't spreading, and the only reason they peel is because I have long fingernails and it peeled away a small bit of skin while I was trying to look closer at a bump.

I just came back from mowing the field again today and found three bumps on the same hand that are the same size as the others, but they aren't in clusters.
I washed my hands immediately to see if it was just some irritation, but nothing happened.

Any clue on what this might be? I'm assuming it's some reaction to a plant but I haven't seen anything unfamiliar anywhere in the field.