Hmmm... someone on here posted a video of kittens moving in mom's belly. It was on youtube. Perhaps a search on there might turn something up. Maybe if you get an idea of the difference between purring/breathing and kittens moving you might be able to tell.
Purring is very rhythmical. Kicking kittens come and go at whim.
On a non-pregnant cat, IMO, it is hard(er) to find their nipples. The fur is covering them and they are so tiny - just a bit more than a pimple really - lol. But as they progress they get bigger, the hair starts to recede (so that they form a ringworm like ring - so to speak) around the nipple, then eventually they will fill with milk. Which will be unmistakable.
Is she eating more? Does she appear to have any vaginal discharge? Does she visit the kitty litter box a lot? Is she nesting, digging or hiding in strange places? At 58 days I think you should be able to tell with at least one sign.
Perhaps she didnt mate on her first heat, or it didnt take. Maybe she mated on her second cycle, which could've been just two weeks later.
Hope you figure it out soon, or that she delivers and you can stop worrying... These cats have a mind of their own!