I understand how concerned you must be for Bella as this is her first pregnancy and it would be very unusual to only have 1 kitten.
You truly cannot tell for sure whether she has more kittens or not without taking a radiograph. Kittens can hide up along the spine or behind abdominal organs and make themselves difficult to palpate. Sometimes a stool filled colon will feel like a kitten as well. I will say it would be extremely uncommon to only have one kitten, especially in a young, healthy cat, but it is possible.
Stage I labor usually lasts 6 to 12 hours, but can last as long as 24 hours. Cats will pant, pace, and be unable to relax or eat normally, even sometimes vomiting. But they are not actively pushing.
Once you can see active contractions, and she is pushing that is stage II labor. Once she starts pushing you should see a kitten within 20 to 30 minutes, definitely within 45 minutes time. If she is pushing with no kitten coming within an hour then she needs veterinary help. It is possible that a kitten is too large, is malpositioned or she may need calcium or hormones to stimulate her uterus.
There should be a placenta delivered after each kitten, so two placentas so far with two kittens is normal.
Some queens may take a break between kittens, sometimes a few minutes or sometimes as long as several hours if she is stressed or scared. This seems to happen more with first time queens. Rarely we can have separate breedings and so delivery dates can be a couple days apart. It doesn't happen often but it does happen. As long as she isn't actively pushing, and can lay down, nurse her kitten, and relax or eat, then this is fine and normal. In general most kittens in the litter are delivered within 8 hours.
But if she is actively pushing or starts actively pushing today and no more kittens follow then she probably needs an examination tonight.
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