Why is the red (common) wire giving me a shock?
This is perfectly normal. One of the two travelers can be hot at all times if the switches are wired correctly.  Which traveler is hot depends on the orientation of the switches.

That said, you shouldn't touch any wire in a circuit box when the branch is live. If you happened to have provided a better ground path, someone else very well could have been asking if it was normal for the red wire to have killed you when you touched it.