It does look like something odd is going on with this boyfriend.
Are you sure it's a boyfriend and not something else she doesn't want you to know about?
But, it may just be that she finally has an adult daughter and needs personal time.
Personally, I'd be very happy for her that she found someone she loves. How many Christmas did she spend with you? She lives with you. Your quality time together should be every day. It's not as if you only see her every week for the shopping.
If you had a boyfriend, would you have spent Christmas with her or him, over lockdown?
As kindly as possible, you are an adult now. Even if you don't, or you can't, live independently from her now, you will. You're likely to get your own home, your own family, even move away from her. She is living her life now, and you should be pleased for her. She has probably made enough sacrificies raising you basically on her own (from your account) so far.
My main concern is why she isn't more open to you regarding her relationship. I'd be worried she didn't trust me, or that it was something that wasn't good for her.
Does she seem happy with it?