First off, to the point of how you say things, it would have been good to point out the fact that it wasn't "her" food that was making you sick but as you said so clearly here, but the richness of it. I mean just clarifying that the amount of rich food lately has been doing this, might cut the sharpness of the criticism a bit. but that being said..

I get the feeling that (IIRC other posts you've made have been similar) you're criticized a lot about hurting your mom's feelings by how you speak or behave and this is just yet another way that your mother can make you feel terrible. I may be reaching here but I'm going to venture a guess that part of your "difficulty" with communication comes from the very person that accuses you of being tactless and/or hurtful and in a consistent manner.

In summary part of this is, yes, being more tactful, but I want to relieve you of some of the guilt you may feel, because I am pretty sure that her response was used as a way to manipulate you.