Help! Is my dog having a panic attack?

It could be a seizure.  It could be many things.  Look at her gums, if they are white, that's sign of a problem. Could something have frightened her badly? Mabye look for an insect bite of some kind.  If it persists for much longer, I would take her to the emergency clinic.  Maybe it's nothing but I tend to err on the side of caution. Please keep us posted.

Edited to add that shivering like that can be a sign of pain. 

She doesn't seem to have pale gums, but it's hard to tell as she has some pretty bad teeth and probably some sort of gum disease. They may just be red from that. =/ I cannot find any bug bites or anything that could be causing her fear...

Her mother (who we also own and who we are pretty sure came from a puppy mill) often has such episodes like this to the point where she has squeazed into closets and defecated all over...

But this dog has never had something like this happen to her. I would take her to our vet right now, but I am on some medication that would make that illegal... -sighs-

I've wrapped her up in a blanket on the couch and am having my other dog lie down with her and she seems a little calmer...but she is still shaking.

Barring any medical issues, which of course would be the first concern, this is sort of how Harry reacted when we had a smoke detector doing the "dead battery" beep.  He was extremely clingy, which he usually is not, he would hide under the coffee table or try and sit right on top of us, and was shaking.  This was for probably an hour after we remedied the beeping situation.  It really freaked him out...I've never seen him like that before, so it was quite scary for me, too!

Maybe that's what it is!
The older dog reacts to that in this way, but for many years, Cassie never has. One is going off every ten minutes or so, though.

I can't reach it, however. ><

I wonder why she would suddenly react to that and never in the past. =/

Can you reach it with a broom handle or something long that you can knock it down with?  Knowing how Harry reacted to it, I'd be trying to bust it apart to get it to stop, lol!

Our other dog, Sammy, didn't care one bit about the beeping, so I have no idea why one dog reacts to them and others don't...maybe someone else has some insight on that.

I tried the broom, but the only button on that thing is too small to be pressed with something so large and blunt. Cassie does seem to have calmed quite a bit now after Denali sat with her for a while. Now she is just sleeping peacefully on the couch despite the beeps.

Thanks for the help everyone!

I'm so glad she's calmed down and isn't trembling.  Thanks for letting us know. :)

And to go right back to the beginning -- such a repetitive noise could actually trigger seizure activity.

If her teeth are that bad she NEEDS to have a dental.  Do you realize that will cut her life FAR short of what it should be??  Yep -- bad teeth = bad heart problems.  Time for a  vet visit.