Why is my boyfriend so bothered that she doesn't care?

Yes im a shitty person. I have been sleeping with a guy in a relationship for about 4 months. He has been with his girlfriend for two years. she's quite a big youtuber.
Anyways, he eventually broke things off with her. And he started dating me a month after. I was expecting her to be sad and angry but according to him. When he told her. She just said "oh, okay. Cheating is a horrible thing to do. But as long as you're happy then im happy for you". She didn't contact him after that or even say or ask about me.
I was relieved, obviously that means theyll be no drama.
He seems bothered by her response. He told me that she was extremely afraid of him cheating in the relationship and her fear was being left for someone else. But she doesn't seem to care. I checked her YouTube. Expecting to find a vlog about the breakup. Its been three weeks and she hasn't mentioned it. Infact, a day after the breakup she posted a workout vlog. She seemed normal.
Im getting bothered by my boyfriend constantly bringing it up. he's been watching her vlogs a lot. And he seems less interested in me. What should i do? And why do you think she responded that way. Most girls get angry
Why is my boyfriend so bothered that she doesn't care?
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