Rash after Botox rare but not impossible!
Botox is extremely safe, but while the risk of side effects is very small, it is not zero. Various things related to the Botox treatment could cause a rash, none of them the Botox protein itself. For example, sometimes Botox powder is mixed with preserved saline, and people can be allergic to the preservative. Some doctors (not many) use a topical anesthetic cream: again, another potential source of allergy.
However, having a rash 6 weeks after the treatment that isn't getting...