Can you help me, please?

Are you asking why she's so different when you two are 'intimate'? It's fairly simple. When you're intimate with someone you let your guard down and some tend to 'go wild' resulting in a complete flip of personality. Like how your girlfriend likes to cuss and being called a bad girl in bed when that would never happen elsewhere.
My boyfriend is like this. He's the sweetest guy and can't even call me a b*tch jokingly (this is common in our friend group so it stands out) and will immediately get upset and apologize if he thinks he hurt me in anyway.
However! In bed it's like he's a different person. He'll call me a slut and what not and be fairly 'rough' not that I mind it though it was a shock at first to see this side of him.
So I kinda can see where you're coming from but if it's strictly only during 'play time' and both you seem to enjoy it then I wouldn't worry to much about it. It's probably just a kink of hers and that's fine as long as both of you are comfortable with it.

Very simply stated, she enjoys being a proper person but she is comfortable with you and she likes to cut loose a bit. Just be satisfied that you get to experience her hidden naughty side.

????

And this is a problem why???? The old saying was that men wanted a woman who was a lady in the parlor and a prostitute in bed. Would you rather have it the other way around?