Girls, What causes acne?

Before I got on birth control pills for my periods, my acne was the WORST. it was so bad that it made my whole face red with all these spots. Then I got on birth control pills for a while and that made a huge difference where I would only get one or two pimples not often. But now for the last couple months or so before I stopped the pills, it's been getting worse again and now I get really bad ones that hurt super badly. Even in weird spots too. They're bigger and they hurt SOO much. It's also tempting to pop them.
Girls, What causes acne?
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