It could well be a hormonal thing, or equally it could be Charley hitting maturity and challenging Darcy. I think you'll have to wait and see how she is once she's out of her season again. Did it start when Charley came into season or just before it?
Caoimhe (2, not spayed) is very domineering towards Tilly (6, spayed); she isn't really aggressive as such now, she doesn't have to be, but she could be at times when she was younger and establishing herself as the boss. Tilly has always backed down to Caoimhe and has never even really attempted to challenge her. Caoimhe isn't really worse during her season, this is just how it is with them, but it did start shortly before her first season, she'd have been about 9 or 10 months old when she decided to let them know she was in charge. I don't know if I'm making it sound worse than it is, don't get me wrong they are far from pally but they coexist absolutely fine 99% of the time, they are both just very clear of their place relative to each other.
I do watch Caoimhe for bullying tendencies, I guess Tilly is a bit nervous around her which isn't particularly nice from my human point of view; if Tills is sitting with me and Caoimhe approaches Tilly will instantly jump down and move away but there's no confrontation, Caoimhe only has to stare right at her and Tilly overtly avoids eye contact and moves away. However, Tilly is not unhappy, she's not a clingy dog and she still gets plenty of attention - Caoimhe gets only individual exercise and training and obviously also spends a lot of time with her trainer. I've pretty much let them sort it out themselves and don't really interfere in their interactions unless it gets out of hand (which is hardly ever). It's not ideal, of course I'd like them to like each other and for Tills to not always kowtow to Caoimhe, but this is how they are with each other and I know from experience it could be a heck of a lot worse; they've sorted it out themselves and as long as they can live together mostly peacefully and they're happy then it's fine.
As an aside, how is Charley's training going? Is she doing well?