I'm voting no because I've seen a lot of mares that have had foals in the past with that type of belly when they weren't pregnant. Since the blood test and scan done by the vet were negative, that would confirm it to me. If your mare were that close to delivering as her size would suggest, the vet should have no difficulty at all determining if she were pregnant.
You don't say how old your mare is, but the older a mare is, the more her belly will tend to sag as the back muscles age. You also don't say how much you are feeding the mare. If you are overfeeding, she may still gain weight even if being worked. Some horses such as my mare and another mare at my barn gain weight mainly in the belly. You'd think the one mare was ready to deliver any day, because her belly is so large all the time, but she's 28 and there are no stallions nearby.
When my mare was overweight she had a belly that moved around and her flanks appeared to flutter out as if a foal was moving around in there. She has never been bred. Udders can also be deceptive since a horse that has had a foal in the past will have a larger udder. Some mares also will have fluid leak out of their udders with hormonal changes or diet changes.
Hopefully you are not letting your horse's appetite determine her size, since if she is not pregnant she will need her diet managed. My horse became overweight because I was letting the barn owner determine her diet, she was feeding my (ideally) 750 lb mare the same 30 lbs of hay the 1300 lb warmbloods were eating, and she said my horse was "so hungry" all the time. That is about double what a horse my mare's size needs to eat. Horses that consume too much hay can be given slow feeder nets to help them not eat as many calories.