Gall Bladder would be my first culprit. I didn't have stones, but I had the pain and my GB just stopped working. I have an autoimmune liver disease and I think I had it then and didn't realize it.
I know you said you've had gas, but it can do some wild and weird things. I thought I was having a heart attack one night due to the pain in my upper side and my middle back and once I passed gas it went away.
The weight loss can be IBS related too. Have you changed your diet in anyway to make the weight come off. When did your dad die? Did the weight come off after that. Depression can cause fluctuations in your weight due to appetite.
As you know, PC is one of those diseases that hides until its really bad. You have loose stools, but what color are they? Does the pain go away when you change positions? If it does, you don't have PC. PC pain (as you probably know) doesn't go away no matter what. My cousin died of it and the only relief he got was a heated blanket and morphine.