My wife (32f) consistently gets nauseous and vomits all night once per month

So for the 8 years we’ve been together, she has seemingly had this issue and some years it’s worse and more frequent. We have assumed it is food poisoning, but it happens far too frequently for that to seem logical, especially since I often eat the exact same things with no issue. She saw a GI specialist last year and did a barium study but it revealed nothing of note. The doctor did not seem concerned at all.

However, the issue has persisted since then and we’re no closer to an answer. Does anyone have any suggestions for what we should be pursuing to figure out the cause of this? Could it really be food poisoning or some sort of food sensitivity?

She is 32 and on birth control but takes no other medication. The issue seems to always happen between the hours of 11pm - 3am and sometimes it is so bad she can’t even keep water down and needs to get anti-nausea medications to control it from an urgent care.

Edit for accuracy: it is not every month but it certainly has felt like it during some periods. There have been stretches of at most 3 months with no issue and it has happened back to back from one week to another before.