need help with friend's kitten

My friend and I both got kittens around the same time (mine is 2 months older) and look after each others when the other couple go away. We've done this about 5 or 6 times. We mistakenly put them together from the 1st time
They were okay, they didn't really warm to each other but would tolerate eachh other. A bit of swipping and hissing.

Now my girl is 10 months and my friends is 8 months. I now have her kitty from week, longer than normal. Things aren't going well and it's only been a day!

Her cat has become increasingly aggressive towards my cat. My cat does follow her sometimes but never attacks her, even when attacked. Now for the first time, she is extremely aggressive towards my partner and I - growing and hissing, attacking us quite ferociously. Not sure what had changed. She isn't eating much or sleeping much. I've been separating my cat to give her a chance to eat but sheven seems really stressed. I think we are going to have to take her back to my friends place ad feed her therr.. or separate them here.

I just don't understand the regression. Should I give up? Start again? I'm aware of the correct way to start introductions to cats.. but these two see each other intermittently and used to tolerate each other and they were getting closer.

My cat has a heart problem too and although I'm really proud of how she is handling the situation, I don't want to stress her out too much. She seems okay now..

Also maybe pertinent - my friend said that she is on heat and getting really fiesty and biting more. I'm encouraging her to neuter her but she hasn't yet. My cat is not neutered either (both females) but can't undergo the procedure due to her heart condition. She did get a contraceptive shot a while ago and I haven't really ever noticed her displaying signs of being on heat. Do female cats also get really aggressive when on heat?

Of course they are strictly inside and both vaccinated etc. We live on a 2nd story apartment and don't even let them out on the balcony.

Would appreciate any advice

Here are the two munchkins


Her kitty


Althoughost both small for their age, my kitten is quite a bit larger too and thus more intimidating maybe?

If it's only for a week and the other cat is going home soon then I would keep them separate. It's not as if they are going to be living together and proper introductions take much longer than a week.  Do you have a room you could keep your friend's cat in? You could let them take turns of having the run of the house so they don't get bored or lonely.

If neither cat is spayed this is probably adding to the tension. All those hormones on top of a new place/stranger in the house won't help.

Thanks. I was thinking hormones might be the driver for the behaviour this time.

I guess I just want them to get along because there will be many more times where they are together. But it may not be possible.

Taking turns with them getting the majority of the house to roam is a good idea. I don't have either one closed off at the moment as they have calmed a bit and are sleeping. So obviously they must trust each other a tiny bit. But I definitely don't trust them together overnight or without supervision.