Do you have much clover in your field? No idea if happens with ewes, but two of my mares have been running milk on and off all summer, probably because of the clover in the sward. I asked a vet about it and he thought that was a reasonable explaination. Apparently clover has an oestrogen-like substance in it (I think..) and can cause lactation.
Alternatively, maybe it's mastitis? Does she seem well?