I have been playing around with this issue for some time, researching posts and sites with no luck and so now I am tossing it out there to the crowd. I seem to get odd symptoms from plant based milks (oat, soy, macadamia nut) when steamed for latte, but not when cold. This happens in two different machines (for oat). Symptoms are mild stomach upset, mild headache and somewhat crappy feeling shortly after drinking and it happens most of the time. My machine is a dual boiler and quite new one with limited steaming use. Water is filtered through a good under sink unit, also new. Boiler temp is about 256F for 1.5 bar, and I always purge and clean before and after. My wife thinks mycotoxins or other bacteria but I know the temp will kill most anything. I guess mycotoxins are pretty survivable but not sure how they got there. I have never seen this pop up in other chats. Anybody else have this issue?

And steamed regular milk is OK?

One thing to consider is that several of those contain things that people have various reactions to, whether you want to call them discomforts, sensitivities, allergies, toxicities, or whatever. Soy and nuts are both common triggers. Carefully noting other foods that don't sit well with you and finding common ingredients may help unravel the mystery.

Don't do regular dairy, but perplexed by the steamed vs cold difference with these alt milks.

Do the same drinks from a cafe have the same symptoms.
If they do, the fault doesn't lie with your home set-up.

Try using barista series milks - they are formulated for steaming.

Could be so many things. Jeff's suggestion to try and whittle down the food sensitivity possibilities could get you there. It might be the additives. If it continues and you can't figure it out though, go see a doctor about it.

Are the symptoms rapid onset and somewhat quick to reside? That would suggest something other than sensitivity to me. If so a couple things might be possible. Food poisoning is something that it could be from your description, a mild case is often just discomfort. Health wise it could be related to sugars as well. I have had a few times I over did the sugar and had symptoms similar especially in extreme summer heat. Definitely can't hurt to make sure you aren't dehydrated since coffee is a diuretic and dehydration can cause similar symptoms as well.

let's debunk the dehydration myth to take it out of the equation; https://www.mayoclinic.org/healthy-life ... ehydration.

I'd start testing the effect of cow's milk, and go from there, if the effect is observed with two machines I'd doubt bacteria are at play.

I have similar experience with hot milk. I have researched online but could not find an explanation why symptoms are only with hot milk. My experience is:

Steamed dairy milk: stomach upset + severe acne
Cold dairy milk: stomach upset

Steamed oat milk: mild acne
Cold oat milk: good

I am not sure why hot milk causes acne even with oat milk... if anyone has an explanation or a link to an article, please share...

Patient: "Doctor.. It hurts when I do this..."
Doctor: "Don't do that."

The actual reason is certainly a topic of interest, but for now your body is telling you something. Listen to it.
You could try just steaming water and drinking that, but the odds of it being the water or the machine are likely zero.
The heat could be increasing the speed at which your body absorbs whatever is causing the reaction and/or where in your body it is being absorbed.