Yes your leo needs a vet trip immediately. It takes awhile for the tails to thin so in the future a vet visit (or at least a fecal screening) is in order as soon as you start to see it happening.
Even if they eat out of a dish she could be eating sand when you are not looking.
Is she still pooping and eating?
It could be an impaction. It could also be advanced stages of parasites. After parasites have been latched on to the intestines forawhile they cause leaks which causes fuid build up in the stomach cavity and can be seen as a swollen appearance.
Which happened first, the thinning tail or the bloated looking stomach?
Either way, she definately needs to go to the vet
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0.1 Bearded dragon
0.1 mexican kingsnake
1.0.2 Leopard Gecko's
0.0.1 Rose Hair Tarantula
1.0 BTS
0.0.1 Reverse Okeetee Corn
0.1 Bullmastiff
4.1 Cats