Yesterday while laying on the couch to take a nap, my cat came up and laid on my chest. I noticed she looked like she had been gaining weight, but was unaware of her being around any males. Suddenly she started 'talking'! I quickly turned her around, and there appeared to be a 'sac' coming out under her tail! I quickly moved her off me (and my new couch!) and brought her into a room used for storage. She immediately crawled into the closet, which had a lot of clutter, so I brought her out and put her in a box with a blanket. She immediately hopped out and went back to the couch! I ended up putting her in a dog cage from the garage, as I have bronchitis and was laying down before work. I knew I would not be able to keep my dogs away from her. An hour later, still no kittens, and I had to go to work. So, I put her in a bathroom with blankets until my husband would be home. An hour later he returned, and she was back to her normal self. 24 hours later, still no kittens, but she is nursing my other cat's 4 week old kitten and carrying him around like her own. I have seen a few contractions, but that's it!

Sounds like she has broken that amniotic sac you saw. If so, it will make delivery harder for the kitten, and thus all the others... there will probably be several more. Sounds like a job for the vet.

Next female cat you get, have her spayed before she finds a boyfriend, instead of waiting for one to find her.
She must go outdoors regularly? It only takes a moment to consummate a relationship, for cats.