What'd she weigh when you bought her, and what does she weigh now? Might be able to get better idea of how old she is. But I don't understand some telling you to sell when there does'nt seem to be anything wrong, but maybe a hay belly. If you bought her as a single, and there was nothing wrong with her that you could deduce, I'm betting she was seen being bred, and rather than messing with her, they sold her. I'd also bet she is bred. I had a Braunvieh heifer calve 5 days short of 13 mos., unassisted, 70 lb. heifer. Christmas day had 19 mo. heifer give birth to 71 lb. bull calf, unassisted. But they were both easy calving Braunvieh.
But I'd keep her and just keep an eye on that potential little money maker. But that's jmo.