Stomach tightness.

Cy
So i'm 20 weeks. This morning I was laying on my back and felt a little short of breath. Of course I knew that was the baby, but when I looked my stomach was tight and had the shape of his body. I know pre-term labor exists. When do I need to be concerned? 
This is also my 2nd pregnancy. I remember the 1st time around when my stomach tightened like this I went to labor 3 days later.