I've never come across either maiden milkers in sheep nor such a swollen whole udder in a maiden ruminant. (The udder is the whole organ, so is always singular - unless referring to the udders of several females collectively. Individual sections of the udder are called "quarters", as they are in cows, even though the sheep usually has only 2!)
I did have a heifer became swollen in one quarter, which the vet called "felon", just clear liquid, no evidence of infection. She had a benign wart on that teat which had pulled the opening open, so we imagined a fly had got in and caused the reaction.
I'd want to get my hands on her and check for heat, hardness, discomfort and yes, milk. Any of the first three she needs urgent treatment (try udder mint if it's very mild, otherwise she will probably need antibiotics.)
When you say there is no chance she's been jumped by a tup... What about unknown fertile males? Could she ever have been kept with an imperfectly castrated wether or a natural rig (one or more testicles internally, could have been mistaken for a ewe lamb when checking... it happens!)? Internal testicles can become fertile in extreme cold; we and many of our farming neighbours had unexpected lambs born on May 10th/11th one year after a real deep freeze (sub -10C) in the mid December previously!