Allergic to What?

2 yo has a rash. I know, kids get rashes all the time. I'm used to seeing them. 98% of the time, with our brood, the rashes are eczema related. I've come to recognize lots of rashes based on where they show up, how the spread, and what the bumps and colors look like.

BUT. A rash that starts as a ring around the neck and moves to the upper arms, avoiding the shoulders all together, is somewhat strange.
So, we went to the doctor's office. Well, our pediatrician is on vacation. The other pediatrician is pregnant and on half days, so we were redirected to walk in. We got to see one of my favorite doctors ever. I don't know how he'd feel about having his name in a blog, so I won't put it. We'll call him Dr. S.  Dr. S is great friends with a local pharmacist and he has a fantastic personality. He jokes about everything, including a possible zombie apocalypse. I selfishly hope, for my families health, that Dr. S is ageless and never ever retires.

While checking out the 2 yo, the 3 yo was up on the end of the exam table, watching everything.
When Dr. S was done, and walking over to the computer for notes, he put his stethoscope on three yo's head, then kept moving. 3 yo gave me a questioning look. So I said it. You know you would have, too. "He was checking for brain activity". I know. Mom of the year, right?!  Dr. S said "Didn't find anything." To which 3 yo replied, (I'm not even kidding here) just like the straight man in a comedy routine should, without laughter, hand on chin, leaning on table "That's funny."  omigoodness!!!! Dr. S and I had a nice big laugh!

Oh, the rash. You must be wondering. Dr. S. called it, in his professional opinion, a 'mystery rash'. Bring her back if it spreads, we don't want to do unnecessary blood work on the little ones.
We went back home, slathered her down in sunscreen and took her outside to play.
Guess what? The rash got worse.
Sunscreen allergy? I didn't realize there was such a thing. I didn't realize how many petrochemicals there are in sunscreen. I guess I never bothered to read the label.
Gosh. Now I'm on the hunt for hypoallergenic sunscreens for her.
Has anyone tried anything they just love? I need to know. They aren't inexpensive!