Constantly wagging spaniel tails are pretty prone to injury,
, it‘s possible that Bella‘s knocked it or got it caught somewhere and may have even broken the bones on the end. I think you‘re really the only one who can decide whether a vet‘s visit is necessary or not, its very hard for anyone to advise without seeing her. If it was me, I would give the vet a call and ask what he/she thinks. I admit to being a little over cautious sometimes but if something is painful and the cause isn‘t obvious, I prefer to stay on the safe side.
Sorry not to be of more help, hope there‘s a happy end to the tale!!