my running form is bad?

samco66

I do cardio and strength training 5 days a week an hour a day. Yesterday i ran 20mins straight ( the furthest i have ever gone) although its not very far it was a lot for me. I noticed that about 10mins in, the back of my shoulders right below my neck was really tense, to the point where it was making it hard to take in really big breaths because it makes it more tense. What am i doing wrong? the only thing i can think of is that its the way im running? i run with my back straight, chest out but not too far out, arms bent at the elbow and by my sides