I'm taking diflucan, (fluconazole) why is my penis tip (urethra) burning? I'm STD free (2 tests taken). It seems worse when wearing briefs and sitting.

Yeast. You must have some type of irritation caused by a yeast in order for them to treat you with diflucan (fluconazole).
Urethral irritation. The irritation can be from simple irritation of the glands at the tip of the penis. It can also be from soap in the underwear or local infection. Try some neosporin on the tip and wear loose briefs or boxers.