Why does my male and female rat keep fighting?

I've had my boy (Romeo) for a few months, he's 14 weeks old.
Two weeks after I got him I got another boy.
Turns out it wasn't a boy, was a girl. (Kindle) she's about 10 weeks old.

When I introduced them they were fine, they've got along until last week, every night he pins her down and we don't know what he's doing, she squeaks really loudly and he sometimes bites her, (not hard enough to do damaged but still hurts her)
Sometimes when I get them out the cage he does it too.
They're fine during the day, they sleep together and everything it's just at night.
She cleans him and they get along, but as soon as the lights are out it goes on for hours.
Romeo sometimes has cuts on his nose and Kindle has a cut near her eye.

Can someone help me with what to do?

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