At wits end with daughter

Hi girls, Please no judgement as this is so sensitive to me! My 3.5 year old daughter’s behaviour has just completely changed (I mean she is like a completely different child) since the arrival of my son 4 months ago. She is extremely angry a LOT of the time at home and she NEVER acted like this. There doesn’t seem to be any triggers either . I have kept her routine the same as before he arrived (she’s with me Monday, nursery Tuesday - Thurs then with me Friday) I am trying everything (husband takes baby to play for some time so me and daughter can have time together but even then 90% of the time she will start going crazy for no reason (or so I feel). I genuinely don’t understand and I am so upset, I feel I’ve done everything right in the lead up to my sons arrival, she helped pick things for him, he got her a present on arrival etc etc and he is also an extremely content baby so doesn’t need constant attention just now. She has really good speech and understanding so it’s not like she can’t express what is wrong. I feel so upset to be honest as some days I look forward to get nursery days because it’s so bad! Me and husband are both very chill parents but now I worry I’ve been a bad parent as why is she acting so badly? I just feel these angry outbursts can’t be normal especially when she was not like this AT ALL before his arrival, it’s like me or husband don’t know what to do with her as she’s like a new child. I just try to give her hugs and positive vibes as much as I can but I have snapped a few times just as it’s so so frequent. Any ideas?