It doesn't sound like a hiatus hernia, you would be able to see it as a lump and you would generally be in pain a lot. If it was it would help to lay on your back where the protrusion would "fall" back into the abdominal cavity.
It sounds like a "stitch" if its under the rib cage. it causes a tightness and a sharp pain which hurts more when you breath in and expand the ribcage?
It could also just be trapped wind which feels uncomfortable after eating or drinking a lot. If thats the case, a small amount of fizzy drink may help you to get rid of it as it will probably help you "burp".
And of course having anxiety disorders doesn't help, but try not to worry getting stressy causes muscles to contract which may also be what you are feeling - a tensing of the body.
If it carries on go and check with the gp :)