Dog gets aggressive toward cats in evening

I got this little lady about 3 weeks ago and everything has been going better than I could have hoped. Until the Sun goes down. She gets along great with my 2 cats. Always going belly up to show she’s a good dog. Then night comes and she growls when they come near her and wants to go after them. (Quick note on the cat…she won’t run!) She knows that cat is the boss and all day treats her with that respect but at the end I’d the day it changes. I assume it’s some kind of separation anexity. What do I do?!