Food Storage

[been1711]

This is the first time I've posted on here, though it's been about a year and a half since I was diagnosed with Celiac Disease. Between a book called The Shopping List and online searches I've been able to stay gluten-free for the most part, or at least when I did have a problem I've usually been able to trace it back to something. There have been exceptions to this however, and I began to see a pattern which may or may not be my imagination, so I was wondering if anyone had any ideas.

I seem to have problems when I use condiments (French's Mustard, Heinz Ketchup) that are almost gone - near the bottom of their plastic containers. I don't know if they pick up something after being in the refrigerator a certain length of time, or if something settles near the bottom, or if there's some kind of residue from the plastic which builds up after awhile. Or - as I said - if it's just my imagination and something else entirely. Does anyone have any ideas?