Usually Basil and Lapka get along very well.
And usually sleep together during the day and night.
She growls at him once in a while when he wants to play and shes not in the mood but lately shes been really different after this:

There's a stray kitty that's been roaming outside, one day the cat showed up at my screen while Basil and Lapka were both in the Lanai.
Lapka started screaming like a bat out of hell, and so I brought them both inside, but then she turned on Basil (who showed no aggression towards the other cat whatsoever-more curious than anything) and started screaming nasty aggressive at him.
This went on for about an hour, Basil couldn't go anywhere near her without her getting nasty. He was kind of freaked out by her.

After about two hours she finally calmed down and things got back to normal.

They've been outside on the lanai a couple times since, and one other time the stray cat appeared, and she had the same reaction.
This week theyve been out on the lanai twice, while the boyfriend was home but not I.

Suddenly this week, shes just started using this nasty aggressive growl almost all day anytime he gets anywhere near her. She's fine with us, but I almost feel bad for him, he looks so sad and worried. They haven't spent anytime together, she seems annoyed and agitated with him. If he gets close enough she grabs him and bites him!

Do you think spotting this stray cat around has made her on edge and irritated with Basil somehow?
I hope she calms down soon, shes scarey when shes mad!

I have had a problem similar to this with BB and Jas. We have a black cat that has been around as long as we have been here, I know he is someone's pet because he is well taken care of with long silky black fur that's obviously kept groomed. He comes around every now and then but likes to come more often than usual the last several months. This poor little lonesome guy has caused pure hell an chaos in our home. Jasper is just now a confident cat, we have had him since Febuary and it's taken him this long to be "normal". Anyways he started peeing in random places through out the house, not everyday but it happened enough times that I noticed he only did it when black jack(as his collar says) comes around. Well come to find out he had been spraying our front porch AND all around the window in my kids playroom. Obviously this caused Jasper to go into a state of pure paranoia and he start taking it out on BB and our dog. He wasn't as loving with us and started hissing at our kids. I honestly was freaking out and I didn't know what to do. I also had noticed that the screen on BB's window where she always lays was clawed. So I went outside with the light that came with Urine Off and it honestly looked like some took a bucket of cat pee and poured it all down our windows,front door, and the rocking chairs on our front porch. I was furious! I had stressed myself about Jasper and the smell of cat pee in my house for almost three weeks, and it was this cat that was causing it. So I cleaned all of the urine off that I could, our house is brick and the porch is concrete so as you can imagine it took several soaks and about $150 in natures miracle to get it off. We set up motion lights on both corners of the house and I checked every night for new areas that he might have marked. Well it finally came down to me catching him in action at about 3 in the morning scratching at the window with
BB sitting there looking at him like he was stupid. So I got a can of food and went out and lured him to me and got his owners number from his tag. I called them at 4 am and told tem to come get their cat because he was terrorizing my cats though the window. It really could have been bad but the couple was really nice about it. He has a cat flap and was leaving in the night to come visit us and everyone else! So they now lock his flap at night and also agreed to let me take him to be neutered. So long story short that cat caused hell for us and since he is no longer coming around everything has retired to normal, even Jasper's tummy problems are gone.

If I were you I would get a uv light and see if he has sprayed anywhere, the scent he has left behind could very well be causing the problems with you kitties!

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Sorry to have rambled you can tell I was really worked up with the situation!!!

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The little monsters 3

Swap their smells from her to him. I use comb -brush her first then the same comb brush him. It may work.
Let us know how it goes.