Do you exercise at all? Sitting at the computer is very hard on the neck -- so is looking at a cell phone a lot -- and the muscles are attached to the area you're describing. But it could have been tweaked just by reaching up to get something from a cabinet or some form of exercise. If you don't exercise, your muscles will obviously not be very strong, and you won't benefit from movement that stretches us out. You're very young, forget about your heart, but do know that if you don't exercise now, it'll gain on you as will not eating a healthy diet. Also, are you on any medications for your anxiety? One of the common side effects is muscle cramps, often because they interfere with the absorption of magnesium, which is essential for relaxing the muscles.
I do sit a lot by my computer in my chair in the same position with my arms on the table and I know this could also be a factor to this and with anxiety that doesn't really help especially when I am terrified of just having a heart attack or something and dropping dead. As I mentioned I do have health anxiety.