Opioids Opiates and weight gain - WTF!?

My question is: do opiates cause weight gain? Not fat but swelling or fluid retention.

I normally weigh around 102lb but found I was hovering around 111lb while on oramorph/MSContin. Despite the fact that I had less appetite and was probably averaging around 900-1600 calories a day. I should probably have lost a good 5lb.
I've been on Oxycodone and OxyContin for just over a week now and have somehow suddenly ballooned to over 120lbs practically overnight.

It doesn't even feel like real weight gain. My legs and face look and feel swollen and my abdomen is grossly distended. NOT FAT. I have a naturally flat stomach being 5"6 and not much over 100lb but now I look a good 4 months pregnant. Not a soft belly, but firm and uncomfortable.

Do opiates normally cause this sort of thing? If so can anyone advise how to reverse or at least minimize the effects? I can't just STOP as I'm physically dependent on around 100mg oxy a day (I take about 300mg codeine a day too).
I'll ask my doctor next appointment as I have a script for everything I take...but thought I'd see if anyone with first-hand experience had any answers?

I do have pancreatitis which may be partially to blame.

This is particularly distressing as I am less than a year into recovery from 10 years of bulimia.

Have you pooped recently?

Yes haha. It's not just a ton of constipation.

I gained 50 lbs over two years from oxy and suboxone. I believe it is constipation, even if you "go" daily, fluid retention, my lack of normal exercise and cravings for sugar/starch while on meds. I too am usually quite thin and this is utterly distressing and ONLY happens when I take opiates and such. I thought I was the only one!

I do know, when I detox and start eating healthy nutrient filled food, the weight starts falling off.

To clarify, in addition to my previous statements, YES, I experience unnatural bloating and spontaneous weight gain that is still a mystery to me.

Opiates themselves don't really cause you to gain or loose weight. It's the indirect effects on your appetite, withdrawal, and things of that nature that either do effect one severely or don't. I've known people who got into opiates heavily and turned into stick figures and others who got fatter.. everyone reacts differently. Everyones body is different.