If you live in an area which gets really hot and humid in the spring/summer, what would you think of someone who always wears long sleeves? Particularly if this girl is skinny (so she wouldn't be hiding arm fat), doesn't have a religious reason, and doesn't burn easily in the sun...
I think she is really insecure about her arms, could be that she has scars there from her childhood, a whole lot of armhair, maybe she selfharms (which could also mean burning herself), maybe she gets abused by someone which leaves bruises. There can be all kind of reasons. But I think the most logical reason is that she selfharms. Maybe you could ask her how she is?
This depends on local norms. I have relatives that live in California where it gets insanely hot (not necessarily humid, but still) and they always wear long sleeves and pants to protect themselves from the sun (and when they work on their ranch).
If I saw this where I'd live (where it gets very humid in the summer), I'd think, "huh…she must be warm" then carry about my day. Not really a big deal, in my opinion.