New bod, expectant or not?

Hi, i am new to this although i have been reading a fair amount of the threads on this forum for a few weeks now.
My dilema is this. My young cat, 10 months old went into her first heat season around Christmas time and managed to escape. I had planned to spay her and will be doing so as soon as is safe for her. I have two other cats 6 years old, male and female both spayed. She went off heat shortly after her escapology and i guessed she may have been caught. I have the capacity to take on another cat and have homes for any others that might be born already. I noticed her nipples pinking up around the three week stage and they seemed to have grown and were four times as big as my other female cat. She has been behaving eratically ever since. She has taken a complet disliking for my other female cat and has decided she is top cat causing lots of screaming etc. I have a big house and have managed a new routine to enable them both to get lots of cuddles and attention as they both love a good fuss. My dilema is that i belived that the younger cat was pregnant and due in the next 10days but she is running around up and down the curtains and is not slowing down in the slightest. She has put a little weight on andsleeps alot but at cuddle time i cannot feel anything obvious (obviously feeling very carefully) I have not noticed much change in her nipples still big and some are pinker than others. Please can anyone help? Is it possible to just have one heat cycle, not get pregnant and then not go back on heat?

I'd have her an ulrasound if you don't want to spay/abort now. Then you will know if she really is carrying kittens. Its possilble its a false pregnancy (it happens sometimes) - in that case, spay her immediately.

If you won't abort kittens, then plan on spaying mom when they are 8 weeks old and keep them for 10-12 weeks before rehoming them. Do not let her out at all till you figure out what you are gonna do.

Keep her separated from the other cats for the time being so everyone can settle down.

When my Meeko was preg she never showed.
The only sign were her nipplesgot pink/red.
She only had one kitten and I kept him.
I lost him last year at age 5 though.
Get the ultrasound.

Thank you for your help
Update, i think Matilda is not as far along as i thought! She has developed fat areas behind her nipples and is putting on weight! she is enjoying alot of fuss on a more regular basis. I spoke to my vet who said not to worry and to keep a close eye on her. It sounds as though it may be a small litter maybe even just one little kitty. I am now checking her temperature on a daily basis and making sure to not leave her for more than a few hours. We have made her a special place for the big day which consists of the puppy cage which is jammed so that only she can get in and out.(i have two other cats, Astrid and Khan and a young dog) I have also put her litter box nearby and she has food and water in the den. I find she is sleeping in there when i wake up in the morning, which may have something to do with the sheepskin rug that is in there at the moment! Oh and it is in my bedroom just in case it is a nightime event. Astrid the cat whom she is being moody with is doing fine, i have restricted there meeting for the time being and will reintroduce when things have calmed down a bit. I am hoping once Matilda is spayed later on it will calm things down a bit. Matilda is being quite affectionate to the dog, Tobin and Khan my other cat and she is enjoying being washed by both which is quite sweet, she has always had fun with the dog and i think she may have a bit of an indentity problem as she copies some of his behaviours bless! I will keep you posted as it is so nice to know there are other lovers of animals out there