Mother cat rejects 5 mth old kitten

Question
We have a 18mth year old female cat, Little, who had her first litter of kittens about 6 months ago.  She had 4 total.  We recently adopted out 3 and kept 1, a female named Squirtle.

When the kittens were first born and feeding Little was an fantastic mom.  However, Little has recently been staying away from our house and really only coming in to eat.  When Squirtle approaches Little, she hisses, smacks her, and walks away.  It's clear she doesn't want anything to do with her.  They are both spay, and up until a few weeks ago were best friends.  Sadly we went away for a trip and squirtle got locked shut behind a bedroom door and was separated from Mum for 3 days, since then Little will not go near her.

Little has always been a homely loving cat, she is still like that to us and our older cat, however she used to be fine with our dogs and hates them also. Also offer reacts to movement and always on guard that Squirtle is coming near her.  
We want to be able to keep both cats in a space that is healthy and happy.  Any thoughts?

Answer
Hi
this behavior is known as trauma aggression and is quite common. it is when one cat becomes frightened by something and blames it on another animal even though it has nothing to do with them.

the getting locked in could have been this trigger.

the way to get over this is to spend some time reintroducing the cats to each other in a safe controlled manner using the introduction method described on this page

/introducing-cats.html

it does take a little time and effort but really does help the cats re build their trust and confidence in each other again.

best wishes Kate