Stomach pain. A couple times a week.

Intermittent stomach pain - hard to describe but it comes on slowly and the pain increasingly becomes more intense. Maybe starts out as a dull pain but slowly hurts so crippling that I feel like I want to go into fetal position. Here is the thing - as soon as I lie down it does away. It comes back if I stand up again too soon. At first I tried to associate it with something... and it seemed like maybe stress but now it just seems random. Mostly later at night so I just lay down and go to bed. Lately I've been noticing flank pain associated with too. Sometimes gas. Nothing else makes it better. I went to the doctor once who tested blood for the typical things but found nothing. It's been going on for years...'sometimes I go months without it. Sometimes it happens a couple times a week.