Any idea how old she is, even just a guess would do

If you had no other male guppies in the tank, she might not be pregnant. If the molly you have with her is a male, it could be possible. Might have been possible with the platies also, but I'm not sure if guppies and platies will breed.
She could just be growing still. Just because there's a gravid spot, it doesn't mean she's pregnant. I heard some females just have them constantly, pregnant or not.
Those signs are just a guideline too. Just because they show those signs doesn't mean they actually are almost ready to give birth. It's just one of those things you notice that means it could be almost time.... It drove me insane with my first pregnant guppy waiting for it to happen, but it did eventually and it's just one of those things you need to keep a close watch on. Every fish is different so yours might just like to hide out in one spot and eat. Just keep a close eye on her
Heh, if she really was pregnant, perhaps she already had the fry and just hasn't lost her shape

That happend to me under 2 weeks ago. I didn't know one of my females was ready until I found a baby staring at me while I did a water change LoL. The female was still just as large, gravid spot just as dark and there were no signs either had even had any fry o.0