Think you're right about the out of date lubricant.
That may have started things off - trust you have ditched it?
I also think that it must be the lubricant. I post below the response I got to my enquiry, quote:
"
If you have had an allergic reaction to Yes oil-based then the best way to check it out is to apply a little to the inside of your wrist, if you are sensitive to any of the ingredients then this patch will show signs of redness or itchiness within about 30 minutes. If there is no reaction then you are most likely not allergic to it or one of its ingredients.
Bouts of thrush usually occur when vaginal pH is raised above typical acidic levels of pH4.0-pH4.5. Vaginal dryness often is associated with elevated pH as the moisture that keeps the protective acid producing bacteria working well is gone and there is no way for them to survive and semen (as it is slightly alkaline pH7-8) can raise vaginal pH after lovemaking. I recommend trying yes water based in applicators to rehydrate vaginal tissues and/or reduce your vaginal pH to its typical levels.
Yes oil-based lubricant contains no water so does not interfere with vaginal pH and it is bacteriostatic, that means no bacteria can grow in it as they need water to grow. Old yes oil-based past its shelf life should not cause any issues as you describe as long as it has been kept out of the sun with the cap tightly closed.
I hope this puts your mind at rest."
We have used the oil based before so I don't see that the wrist test is relevant.
I have of course ditched the offending out of code bottle
