I am not sure its the vitamin K2, whenever I take it I don't feel anything. From what I understand is that it helps to utilize calcium in the body so it doesn't harden in the arteries, the Japanese have a diet in Natto which has a lot of K2.
The Japanese also have a high diet in Iodine too, both have been said as the reason they live the longest.
I also hear its good for diabetes and Sugar issues, but as for the stomach problems maybe it's giving you constipation if you have a diet high in calcium like from eating pizza and a lot of cheese. But again, it should help move that calcium around to the bones, so its probably excreting excess if you have high levels. Maybe lower the dosage, it's a good supplement all around the internet.