Agressive behaviour female cat

Hi there, I have a 4 year old female brown tabby. We adopted here a year and a half ago from the Humane Society.
She has always been a very calm and agreeable cat with only the odd nip if she was being "overpetted".

Recently she has become very aggressive with me (I am female), nipping me when I try to brush her (she never did this before) and now hissing and trying to bite me when I pick her up or try to put her harness on. She does not do this with my boyfriend.

My boyfriends’ theory: There is a male cat that lives next door that has been passing through our yard right by the window where she sits and she gets EXTREMELY agitated by this, howling and hissing. I am often outside when he comes through and since he is very cute, I often pet him and rub his tummy and she can see me doing this. Is it possible that she is angry at me for interacting with this other cat? I find this hard to believe.

I am very saddened that she is reacting to me this way since she used to sleep on my lap and let me cuddle or kiss her. Now she is growling and hissing at me even if I try to pet her!! She is also obsessed with sitting in the window he passes by and will only leave it to eat and use the litter.

Help!!!!!

Is she spayed?

And yes, it could very well be jealousy!

Yes she is spayed.
We always bring her outside with us in the backyard and she used to just lie there and relax. Now she is crouched and stressed looking and tries to bite me when I put her harness on or take it off.
Is it also possible that she might be sick? She is still going to the bathroom and eating and drinking as before.......

You could take her to the vet as a precaution, but I have definitely experienced a jealous cat before. Thankfully, the one I was dealing with (long time ago, before my cat allergies kicked in) would just hiss at me and swerve to avoid attempted petting. They were both cats belonging to the family I babysat for, so I wouldn't think Boo would be so possessive, but man, he was!

She may eventually get over it ...

Well I really hope so. I guess I will have to avoid interacting with the other cat and try keeping him out of our yard and away from her window. I will see if this helps to relax her.
Also, winter is coming soon along with about 10 feet of snow so that should put an end to the other cat's roaming.

Hopefully she will settle down. This is not boding well for my dream of adopting a second cat. She would probably tear it to shreds!!!!

Thank you for your help.