Flexing biceps to the point that it hurts stupid?

Flex away. If it's massive, you're squeezing the blood out of it, and this will make it briefly sore. You might also be putting a little bit more strain on the tendon and getting a bit of bleeding there where it joins bone. But what the heck -- showing off is fun and if you haven't ruptured a tendon lifting, you won't rupture it flexing.
Most likely! Painful activities should be avoided. It could lead to injury expecially with the use of heavy weights.