Can anyone explain why my mother at 92 has "morning sickness." She has a good breakfast and shortly thereafter is weak, faint and lightheaded?

What is her routine breakfast foods? If it’s fried eggs, fried ham or bacon, heavily buttered bread, etc. she could be made nauseous from the grease and then it passes. Lightly sugared oatmeal with a pat of butter might agree better. Dry toast? Leave off the citrus juice but cranberry or apple would agree.

I’ve had nausea all my life. I had my gallbladder removed and that helped. But I must stick to low grease food. Try some experimenting and see.

This is a duplicate question by Willy that has 4 answers:

https://www.agingcare.com/questions/my-mom-gets-morning-sickness-has-anyone-else-experienced-this-with-a-parent-435893.htm

Potassium, Iron or any pill like that in the morning?

Willy it sounds as if you are relatively new to caring? Is this new?
Does she do this at times other than after eating?

i wonder if your mom is taking medication that should be taken on an empty stomach?

As for the bowel movement issue, the elderly are obsessed with their bowels. I could give you 3 examples off the top of my head when I've had experiences with elderly people who obsessed about their bowels. I don't know what this is but it is apparent in practically every elderly person I've known beginning with my own grandmother almost 30 years ago. Someone should conduct a study....

Addressing only your first question as to the post meal weakness, some possibilities are an improperly balanced meal (i.e., not enough protein or iron and too much sugar, including in any fluids she might drink), response to a blood pressure med to lower her BP (which can cause orthostatic hypotension and weakness), or some combination of meds that could create a similar effect.

Has this happened after a change in medicine, or other condition? How long does this weakness, etc. last? How recently has she had blood work to check for anemia?

Is she having trouble with digestion? How is she like after lunch or dinner?