I think I know what you must be going through, Rosanne. I had a cat called Arthur who I adopted from a friend who had developed severe allergies. Arthur would get into vicious moods. He would literally dash from one end of the apartment to the other, leap up about shoulder height and propel himself off the wall! I was terrified of him, and tried the spray bottle method to no avail. I even tried putting him in the bedroom to calm down on his own. That seemed to work temporarily. He would go after anything that moved, usually my ankles.
Eventually, a stray wandered into my neighbourhood and I ended up taking it in-the companionship helped his behaviour a great deal. In fact, that's why I kept the stray, who was gorgeous and would have likely been adopted right away from a shelter.
I'd suggest that your Sammy may be just bored-I know how much Simon's arrival helped.
I should add, however, that when I moved out to B.C. with the cats (I by then had one more, making that 3) Arthur did not adjust to the new apartment, which was a bit smaller than what he was used to. He now lives with my mother and her 2 cats and 2 dogs, and he gets to go outside, which was not permitted when he lived with me as I live in an apartment. He was just always really wild. He is now really pretty happy, he goes in and out and seems to have a pretty good sense of his surroundings in terms of safety.
Sorry for the novel.
I hope your vet appointment goes well!