One of the primary causes of kava grog causing stomach upset is that you ingested solid root matter. The bark on the roots is a particular culprit to this problem as some vendors may not spend as much time as others peeling the roots before drying. But generally speaking, root solids aren't very digestible. You may want a smaller micron strainer if you're using medium-grind kava. Aim for 100 microns or smaller. If you make your grog properly with a 100 micron bag and you still get stomach upset, consider buying a smaller micron bag. Check your bag as well, as there may be a hole in it which may be how solid root matter is getting into your grog. No two strainer bags are exactly alike. As kava is meant to be kneaded, you'll want a strainer bag that can handle a bit of punishment from squeezing, grasping, and general rough-housing. You may be lucky and find a cheaper strainer bag that fits the bill or you may be hurting yourself by spending less on a strainer bag specifically for kava. Check reviews both here and on online retailers that may sell strainer bags.
If you're drinking micronized, micronized is known to cause stomach upset in higher quantities, particularly if the roots weren't thoroughly peeled before they micronize it.
I don't know your specific product and prep, so I can only guess at what problems you may be having.
Switching to a different kava cultivar may not solve your problems, especially if you're buying from the same vendor where they may give equal treatment to other roots they process. It may not even be the vendors' fault but rather the distributor. Check reviews of particular products and see if nausea is a common complaint. Make sure you're not using a faulty strainer bag if you're doing medium-grind kava. If you're drinking micronized, try spacing out cups of it rather than drinking a big cup at all at once to see if nausea issues alleviate before you conclude that a vendor may not peel its roots.
