I haven't seen Higurashi in a pretty long time, but I believe those are infected wounds she attained by violently scratching herself. If you watch the anime, you'll see that due to a virus called the Hinamizawa Syndrome, Rena has episodes in which she thinks there are maggots crawling around in her skin, and she desperately tries to scratch them out. (the virus is dormant in all citizens of the town, but for some reason that I forget, is active in Rena in some of the story arcs) I hope that helped, if you decided to read the spoiler~
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