So, she kept 2 down, but barfed on the third one? How often is she fed? Weekly? Once she chucks, you gotta give her some time off before offering food again, to allow the gut flora to reform. Trying to rush it will only make the problem worse.
The two most common causes are low temperatures and handling too soon after feeding. Parasites (crypto) can also be a cause, but seem to have been ruled out here, along with temperature (what are the nighttime lows?).
Another possibility is constipation, does the snake in question go regularly? Can you feel any lumps in the snake's lower 1/3 of the body?
Other less obvious possibilities include obstructive lesion of the digestive tract, either inside (e.g. foreign body ingested or inspissated (thickened) stool) or outside it (abscess, granuloma [a mass or nodule of chronically inflamed tissue with granulations that is usually associated with an infective process] or tumor). Injested toxins can also cause regurge.
Is the regurge blood tinged? If so, gastric erosion and/or ulcers could be the cause. Endoscopy may be required to detect same.
Hope some of that helps...
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