cats not getting along

i've got 2 cats, Yama 3 and Thumper 9 months. They have always gotten along really well, yama seemed to mother thumper alot when she was younger. We moved a month ago and over the last few weeks the girls have enjoyed going outside, which they have never done before. Yet over the last week or so yama has been growling and hissing at thumper occationally. I thought it might have been because thumper wasn't desexed, so i had her desexed yesterday. Since thumper has been home all yam has done is hiss, growl and hit her.
I am really concerned because yama has not presented this sort of behaviour towards thumper before, is it the move? Being allowed outside? Thumper being desexed and having the scent of the vet and other animals on her?
Any advice on this matten would be appriciated

The smell from the vet is likely the cause, and it'll wear off soon, but wipe both cats with the same towel (back and forth) to help them smell alike sooner.