I think you are doing absolutely the right thing by taking Sierra to the vet specialist tomorrow. The changes in behavior would worry me, too. When you have a cat with special health circumstances, you have to be attuned to these things. Cats are so subtle sometimes (well, at least, "subtle" to humans) in how they communicate their problems to us.
I know that taking Penelope to the vet specialist who is 15 minutes away is torturous for both of us, so I can imagine how stressful a three hour trip must be. I also know, like me, you will probably have to drive with one hand most of the trip. Penelope will not stop crying unless I put a hand in the carrier to pet her. It's funny because even when I take my hand out for a couple of seconds she starts crying again.
Anyway, I am very worried about you and all the stress you are undergoing lately. I know tomorrow is going to be a very difficult day for you so I'm sending many many prayers your way.
P.S. Penelope and Molly wanted to tell Sierra that she shouldn't be scared because her mommy will be with her every step of the way.