Chances are it is. But more accurately, it's also taut.
This means your Trapezius muscles are stretched taut - beyond their natural relaxed length.
Why do these muscles end up in the overstretched position?
As you can see from the image, the Traps cover a large area - all the way from the occipital ridge at the top of the neck down to the mid and lower back.
Different sections of this muscle perform different, and sometimes opposing, movements. For example, upper traps raise the shoulders, while lower traps pull the shoulders down.
However, one thing is always the same for all Trapezius fibers: they over-stretch in a prolonged seated position.
After sitting for a long time, these muscle start to lose tone and go from stretched to "taut and weak."
Once that happens, they develop areas of chronic tension - knots/trigger points in the middle of the muscle and at the attachments.
Maintaining good posture becomes very difficult. The traps habitually go back to their overstretched and weak/taut position.
When you grasp your own traps over the shoulder, they sure feel tight, and full of tender spots.
Is it a good idea to stretch them?
No! -- They are already overstretched.
This seems counter-intuitive. Many people, including well-meaning yoga teachers, recommend all sorts of neck/trapezius stretches. When we touch them and feel their tension, our first natural reaction is to stretch.
However, stretching the traps will make these muscles worse because stretching is what caused the traps to be so taut and painful in the first place.
What to Do to Fix It?
1. Strengthen the Traps (and other upper back muscles) on a regular basis - minimum 3 times a week. Ask a fitness professional about how to.
My favorite strengthening exercise for the Trapezius is the Cobra/Superman(woman) variation.
It engages upper, mid, and lower fibers. It's best to hold it for a minute breathing comfortably.
2. Get a massage. Neuromuscular Therapy, Deep Tissue and other modalities are very effective at releasing trigger points and relieving tension in the Traps.
If you have specific knots, a skillful therapist can take care of those in a single session.
3. Roll it yourself. Pressure relieves tension. That's the basic principle behind massage. Get a tennis ball, or a Lacrosse ball, and lean back against tight/taut areas of your back. Refer to the anatomy chart to hit the right spots.