I think that's a totally normal pattern - some nights maybe she just needs to tank up more to get enough milk to sleep.
I found my DS was happier when I stopped looking for other reasons why he might be fed up and offered a bf first. I would then check his nappy etc if that sight seem to work, but 99% of the time he just wanted a bf.
Cluster feeding can happen at any point, afaik. My 3 tended to do it most in first 4 months or so in the early evening-late nights. I suppose it depends, ultimately, on your baby how much and how often she wants to feed. As long as you're giving her every opportunity, responding to her cues, you'll both be fine.
Mum to a 4 week old bf baby and im confused about cluster feeding. My baby wants to do it a couple of nights a week but i had it in my mind that they only did it for growth spurts at 3, 6, etc etc. Can someone please mmake it clear for me? Or am i just giving her boob when she actually wants something else? We run through the checklist until only option left is food, which she more than happily takes even if some times are a little less enthusiastic than others.