My uneducated guess would be insect bites, caterpillar spines, or an allergic reaction to something generally innocuous.
GH is pretty distinctive and is only really similar in appearance to common hogweed in the UK. Cow Parsley is much smaller with slender fern like leaves.
The main difference between the two hogweed is size. Even an immature giant is a hefty plant with large robust leaves and stems. Other than that the shape is different, but that's hard to describe simply. GH has rusty purple/red spotting on the stems CH has purple blending into green, any spotting is very minor.
The difference is akin to that between a panther and a domestic cat. They are superficially similar but on a different scale, plus some subtle shape differences.