@tamsin

In a shared kitched it is likely to be a contamination issue, but check for weird things like new toothpaste, soap bar, paper straws, dry shampoo.

Lentils (and pulses in general) have a extremly high risk of contamination due to harvest methods, same goes for flax and hemp seeds. Lentil crackers, soy "meat" - anything like that could be a problem if not certfied gluten free.