It’s difficult to say what could be causing the issue without being able to ask you some questions and doing a physical exam. What I can do is give you a bit of an idea of some of the things I have seen in the clinic that can cause itching like that.
STI related I have seen pubic lice causing itching like this. I have also seen non-STI things like yeast infections cause these symptoms as well. I have also seen situations were it looks like something irritated the skin and caused it to itch initially, but now the continual scratching keeps irritating the skin causing it to always be itchy.
You have a good plan of going to see a health professional for a STI check up. Just mention the symptoms you have outlined above and see what they say.
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