i can't tell if I'm really shy or if I have social anxiety.

Shyness is a personality trait. I think we've all felt shy at some point. A few butterflies in the stomach hardly compares to the mental disorder that is social anxiety. It's debilitating and impedes some part of your life e.g. socially or career-wise, with anxiety lasting weeks prior to an event. There are also panic attacks, which make you feel like you're going to die, even when in your head you know you're not.