Henry of Huntingdon makes her the daughter of Coel, King of Colchester; the "old King Cole" of our nursery rhyme, and as mythical as other eponymous heroes. Bede calls her a concubine, a slur derived from Eutropius (A.D. 360), who calls the connection obscurius matrimonium (Brev. x. 1).
Eumenius, 'Panegyric on Constantine,' c. 8.
Eumenius, 'Panegyric on Constantius,' c. 6.