If you're not satisfied with the results of the STD test, you can always ask to have another, especially if you or any partners have had any other contacts—or you suspect they MAY have had other contacts—since the first test.
If STDs caused loss of libido/sex drive that quickly, they wouldn't spread as easily/much as they do.
IMO it's more likely that lack of libido is either hormonal (including pituitary) or medication-induced in nature.
medications that can affect sex drive
Doc