She's not fat or extremely gravid. She looks very healthy and maybe about maybe a week into gestation unless she is younger and smaller than she appears to me. Because you said she had only ~3 babies a _ weeks ago - she may be hitting her first big growth spirt and your noticing that. Getting them child bearing hips happens quick! Make sure to add some real moss or fine leafed plants for the babies to hide in so they dont get eaten : IF she's eating them. Usually the only time a female has 3 babies is her first litter and then will only eat her babies if she doesnt have a consistent food source to back her maternal instinct to eat consistently to produce all the babies she's got to. Would definitely pick up some Christmas's moss or hornwort for the babies to hide into and observe the world before getting into danger.
Your guppy is healthy! And if your guppy's gravid is black it's stomach have many guppy fry that are still small. And if your guppy's gravid is red (like yours) that means she is ready to breed.
Hopes this helps!
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