R.J.
2 things are EXTREMELY common:
1) A strep rash following what appear to be cold/flu like symptoms, but was actually a strep infection. ((His blood work has shown active antibodies for strep, which either means that it WAS a strep infection, OR he's a strep carrier -not a big deal- 80% of hospital workers are staph or strep carriers, and everyone's a carrier for *something*. A person who is a strep carrier is unaffected by strep, but they always test positive for it, regardless of what they're sick for. Strep carriers are common, so finding the antibodies isn't definitive, although many doctors would assume such. Sounds like you have a good doc that they're also investigating other possibilities along with the most likely culprit.))
2) Allergic reactions to antibiotics. Also super common, they USUALLY start as symptoms so mild you miss them, increase over time to noticeable symptoms, and then either the allergy vanishes or it turns into a full anaphylaxis allergy. If I'm reading your timeline right, the amoxicillin was started AFTER the reaction... so if the rash didn't get worse from the amoxicillin cross allergy off the list.
Juvenile Arthritis is less common than both of the above, but it ALSO fits the symptoms. (Actually so do a whole host of other things, but those 3 are the best fit. The first two being really reeeeaally common. Without testing, or another case/flare up it will be impossible to know which.)
Since the benedryl had effect, that tells the doctor a WHOLE lot, but it doesn't actually give a diagnosis yet. JA is an over reaction of the immune system... it starts attacking the body, instead of the invader, typically at the joints. The same way that an allergic reaction attacks a substance as if it were poisonous. The body's immune system is turning against itself. Benedryl having effect could be EITHER helping the RA or the inflammation from a strep rash or an allergic reaction to the meds.
It sounds like you're receiving a high standard of care, BUT next time the rash returns, get medical eyes on it pronto (trained eyes are worth a thousand words). Either at the office or the ER if at night, until the rash has a Dx.
best
R