The caterpillar is about 2 inches long, it has protrusions that end in circles of red/purple hairs.
Your caterpillar is one of the crescent caterpillars that turn in to butterflies. Without photos from more angles and the head, it is hard to know for sure. It is probably the caterpillar of the pearl crescent butterfly.
These are common butterflies, tho the caterpillars are not seen very often. As large as yours is, it was probably heading to pupate.
The pearl crescent caterpillar feeds on wild asters. The butterflies eat nectar.
Crescents in general, as well as just the pearly crescent, are found throughout most of North America. They are found in gardens, meadows, fields, roadsides and woodland edges.
These caterpillars are not known to sting or cause a rash.