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The expression is I'm taking a bath. Use the present participle to talk about an action that is taking place in the present. Thus, you can say I'm taking a bath but not I'm having long hair. That is because having long hair is not something you do.
I'm taking a bath is a tempororay activity, while long hair doesn't change that fast. If you were going to have extensions added for an evening and then removed, you might say 'I'm having long hair at the party', but this is highly unusual. If you are talking about your natural hair, then we regard it as a permanent state.
I have a grammar question but I don't know how to answer it. 
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