Hey 73monte...this sounds like something I had three times now. Pityriasis. You are only supposed to get it once in a lifetime---and when you are young (I am counting when I got it in my 30s as still being young then!)
It starts with one red patch usually on the chest sometimes on the lower back or belly. It then spreads to the torso only--occasionally the upper thighs or shoulders. I thought they looked like big mosquito bites I get. Benadryl did not help me except maybe it reduced the itching???? I was told to use OTC corticosteroid cream---which really only made my clothes have greasy marks. Mostly I had to wait it out.
I have also had something called fifth disease (another childhood disease I ahve had at 12 and 35!)which looked similar but spread to my legs as well. I am the land of wacky skin issues it seems
Finally, I am allergic to pine tree needle pricks, but not just having it in the room so I am doubtful this is that.
I would take pics of the welts now but get in to see a doc just in case. Ask about pityriasis and fifth disease.