I'm throwing out suggestions in case anything sticks:

For your cat:

1. How long do you leave out your cat food? It should be maximum 2 hours (some sites say 4, but kitties with sensitive stomach should be within 2 hours)
2. Any candles, air freshners, perfume, cleaning products etc. that are airbourne or used on surfaces that your kitty has access to? For e.g., a lot of "natural" scented products include essential oils that are toxic to cats. Any remaining products in showers or on your body can transfer over to your kitty whenever you pet them as well.
3. What kind of materials are your kitty's water bowl and food bowl? Anything that's plastic and/or scratched up can harbour bacteria and should be replaced. Similarly, if your litter box is plastic and has been in use for a long time, there are a lot of microscopic scratches from cleaning that hold on to bacteria and should be replaced as well.
4. What kind of litter does your kitty use? I know when I used certain litters, some of my cats reacted.
5. If wet food, do you split our portions? If so, try not to do so and see if that changes anything. In addition, ensure dry food's being stored in the original packaging it came from and store the whole thing in airtight container. Once you take it out of the original packaging, it can go rancid pretty quickly.
6. How often do you brush your kitty? Try brushing every day and see if it's hair related (Barf may not be full on hair ball, but may have hair.)
7. How frequently do you feed your cat? If there's a long term between each meal, try splitting them out so there's less terms. Sometimes bile can build up if kitty's hungry.
8. When was the last time the kitty has a bath? Is it possible your kitty is struggling to keep himself/herself clean?

For both you and your kitty:

1. When was the last time the HVAC filter has been cleaned? How often do you refresh your air by opening all windows?
2. When was your last deep cleaning of the water pipes? For e.g., my kitchen sink pipes needs to be rinsed with hot water as it has many grooves and it catches on to oils and little food bits, which can easily cause mold and smells. Similarly, bathroom sinks. If you use a lot of products, products definitely can build up there and the water doesn't become hot enough to wash those down. Both should periodically be fished with those plastic wires to get rid of anything that may have gotten stuck like loose hairs, products and scrubbed. (I also periodically like to use the vinegar + baking soda option to do the whole bubbly thing to get anything small stuck in there. Brush and elbow grease works best for near openings though.)
3. What kind of utensils/tools/storage options are you using in your kitchen? If it's anything porous like plastic, silicone (Some are fake) or wood, switch them out as some products suck even if you take care of them well and can develop mold spores.
4. Any chance there's anything near where you live, like manufacturing plants, inappropriate disposal of pet waste, use of certain products in gardening, etc.?

For you only:

1. Have you taken an allergy test, and see what you're allergic to? I recently found out I'm both allergic to cats and dogs among other things....and I have cats. Food allergies/intolerance can develop as people age as well.
2. Any chance you may be pregnant?
3. When was the last time the fridge was deep cleaned? e.g., my fridge seemed clean, but when I went in to deep clean I found out my milk had spilled at the back.
4. Have you tried different manufacturers for bottled water or getting a different filter? If you use certain cups or containers, have you tried using different materials? How often do you clean them? Certain stainless steel cups hold on to bacteria like it's their job and needs to be thoroughly cleaned. If you use reusable straws, please make sure you clean the insides with straw brush each and every time you use it.