I would bet my last tenner it's not a dominance thing.
It's very hard to know without seeing her in real life but my guess would be along the following
She is scared of other dogs. They worry her - which may be linked to the ball, but may not be - and she has taken the view that 'attack' is the best form of defence - probably because she gave signs of anxiety earlier on that were missed and so she has restorted to increasing her behaviour to this rushing/barking which - almost certainly - works because you then step in and help her keep a distance.
The daycare is a red herring, I think. This is a small environment she knows very well and - most likely - with some or all of the dogs being dogs she knows well. Just like you can be scared of crowds but love your family, she can be scared of strange dogs but love the ones she knows. Actually, I've know quite a few spaniels that are exactly this way. At day care she is in an environment where she feels confident and with at least some dogs she knows well so she can take cues from them that there is nothing to worry about.
A vet check is a great start, but please come at this from the angle that she is scared and needs distance/protection from other dogs (including a need to be walked where other dogs are not/are kept at a great distance). If I'm wrong you can do little damage this way round. If you assume dominance and are wrong then you can do a great deal of harm but attempting to punish her out of her behaviour or force her to be around starnge dogs more to 'get used to it'.
It really is wsorht engaging a good, science-based trainer to observe and help point out what they think is happening here. Nothing beats an impartial real-life observer with knowledge of dog behaviour, to help you understand what the dog is really doing and what you may, accidentaly, be doing to make it worse (if anything).