I really think there is a combined effect of fish oil and aspirin on your stomach.

I've taken aspirin seperately from fish oil and had no problem.

I've taken fish oil seperately from aspirin and no problem.

However when I took both, and for awhile there I thought well Fish oil was a very benign thing to take. So it shouldn't count when having a problem, You always should be more cautious than me, even with fish oil.

First of all fish oil can cause stomach upset or heartburn and so can aspirin, but somehow when I take both, it causes some kind of fatigue, alongwith the heartburn.