This has happened twice now, after fooling around in my woods (I like wilderness survival things), I've come back home with tiny red dots on my arm, the first time I had them they were right below my left elbow, they didn't bother me at all and went away in about two days, I once had a run-in with a cactus at school and the red dots I contracted outside kind of looked similar to the ones I got from the cactus (though the cactus ones lasted an entire two weeks).
Today I was in the woods, I made a fire, which required touching a lot of branches/sticks on the ground, leaves, cutting a few branches of pine, etc. I then noticed a bunch of tiny pinpoint red dots on my left wrist (see picture), and came inside later and washed up. About 6 hours later (now) they are almost faded and gone. This time I got the dots I actually felt them sting a little, mostly when they came in contact with water. But they definitely didn't "hurt" or irritate me that much, this is more of a suspicion than a health concern I suppose.
I've only had these tiny red dots twice, and both times were after touching plants in my woods. I live in Vermont and we don't have that many dangerous plants, I would've assumed they were from the milky sap from the ranunculus repens covering my field, though the second time I got them (today) I was nowhere near the ranunculus repens.