Stomach Problems, Please Help?

For over a week I've been having pain in my upper abdomen, right in the middle. Sitting is uncomfortable, standing is hard, but when I lie down the pain completely goes away. Upon standing I feel as if there's a weight in my abdomen. 'Holding' that area helps. I've also terrible, terrible gas pains, primarily in my back. I'm able to belch some but it's not enough to give me any relief. I've tried every OTC simethicone there is and nothing touches its! (The next symptom is a little gross.) I'm mostly constipated, but when I do go it's a yellow color. Years ago I was diagnosed with IBS, and I don't eat as well as I should, but I've not had a problem like this before. The gas pain is what gets me the most. I've never had this much in my life and I can't get rid of it, or my body just keeps producing it. Does any of this sound familiar to you? I've looked up everything and no disorder has all my symptoms accept for IBS. If that's what it is I don't know what has caused this very painful flareup.