There is a definite possibility that the infection may not have been completely eradicated with the previous treatment. CDC recommends rescreening at 3 months post therapy for sexually active women, as they have a high rate of reinfection. Please see this authentic resource for reference;
Thus this may not be new infection
and an older one may be persisting without giving symptoms earlier which is not uncommon with the trichomoniasis and your husband denying any new contact.
Both of you would need a new course of Flagyl or Tinidazole and a retesting would be needed after three months to confirm that infection has been completely eradicated.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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