37 weeks, mood swings and feeling guilty

Hi lovely forum members.
Can I vent for a moment? I'm almost 37 weeks pregnant with our 2nd child; we have a daughter who's nearly 4 and after a long time trying, we're having a boy.
We're so happy about it, but I've noticed that since this week I'm having huge mood swings again and feel so incredibly guilty towards my daughter.
Last week we had a super fun holiday week together at home; the daycare provider and family were all on holiday so she was lovely with me. I was totally relaxed and our daughter really lit up (I surprised myself because I'd never been so chill, it was really nice!). We didn't have any trouble at all, but this week I've completely lost it. I'm not in the mood for anything, I'm short and do little with her.
Luckily she can play on her own well, but I notice I'm keeping to myself. I also tend to snap, and of course she gives it back to me. I feel so guilty about this and try to think about it, but it feels automatic; I'm just grumpy this week, lazy, and have little desire to do anything.
If we didn't have another baby coming, I wouldn't have cared, but now it feels like she's the one suffering because I'm at the end of my rope.
Who relates to this? Any tips? Or is it just 'normal' to have these mood swings and be more withdrawn?