You can't prove that it wasn't you. Since the password wasn't changed, it adds even more suspicion towards the culprit being you (though it could have been the potential hacker's way of infiltrating people's trust undetected). There is no way to prove that an account was hacked, and likewise there is no way to prove that an account wasn't hacked. However, I'm leaning more towards 'your account was not hacked'. It was suspicious of your posting quality (grammar, structure, punctuation) to chance from the hiatus between July and December, though that's still not an indication of hack (but it does help your case).

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What made you decide to revisit the forum all of a sudden, though? That seems pretty suspicious (even if this was a month after the scams)