A knot in my stomach: biologically what's going on?

We had a traumatic event happen last night. I won’t go into details; but I will
say that it has left me with a knot in my stomach.

We’ve all experienced this at some time (I dare say). But what exactly is going on
inside when you feel this way? Is there a tightened muscle? Is it an excess of stomach juices? Is it all in the mind?

Anyone know the SD?