I had a root canal on tooth 13 this morning. this evening it feels like i've pulled a muscle in my neck and left shoulder. are the two related?

No: No not specifically but if your mouth was open or your head was in an awkward position during treatment for an extended period of time I guess it might be possible. Most likely not but I'd take a hot shower, massage it out & so ibuprofen to help with inflammation & pain.

Speak with DDS: You may be sore from having your mouth open. Speak to your dentist to prescribe an anti-inflammatory medication and see if it gets better.

Might be: Keeping your mouth open for an extended period to allow access to the tooth may have caused your symptoms. The actual root canal procedure would not.

Possibly: You may be sore from sitting so long in one position while the dentist was working. It'll go away soon.