Veins transport blood back to the heart. They cannot experience pain since they don't have nerves.
Most likely you stressed a tendon.
Happens all the time to me. I type all day, lift weights, mountain bike, road bike, ride a motorcycle, etc...
The things I've found to watch out for:
- never hard stretch a cold muscle
- stretch your arms softly during the day while typing
- stretch your arms while playing guitar (between songs)
- don't lift for muscle bulk if you are actively playing
- give your arms time off