Whenever you complain about something I always think that you're complaining about me. But, maybe it's just a misunderstanding and you're just venting and grumbling by yourself.
ご質問ありがとうございます。
「文句を言う、と愚痴を言う。の違い」は英語で「difference between complaining and grumbling」と言います。
また、「あなたが何かについて文句を言っている時、私に文句を言ってると思ってしまう。でも、もしかしたらただの愚痴を言ってるだけなの?」と言いたいなら、そうすると、「Whenever you complain about something I always think that you're complaining about me. But, maybe it's just a misunderstanding and you're just venting and grumbling by yourself.」という風に言えます。
ご参考になれば幸いです。