Thanks for your question and welcome!
If you were given Flagyl for Trichomoiasis, it is either given in one day or 7 day dosage. In your case, it was most likely the seven day dosage. If you were already on day 5, chances are that the medication had already done its job-- a follow up is necessary to confirm, though. Since you also used a condom, this also reduces the chances of you passing on any STD to your fiance.
There's a small chance that you may have passed it on, since we don't know if you were still infected-- but it's very small and you did the right thing by using a condom.