So our water isn't horrible but it's a bit on the harder side. I see mineral deposits on shower heads etc, but it tests just slightly over recommended hardness.
I purchased one of those water softener pouches but I noticed a strange thing. With the water softener pouch in the tank, I feel very bloated after drinking my morning coffee. I initially didn't relate it at all but started to thing that was the only difference in my normal routine. When I removed the pouch I returned to 'normal' again. I've done this several times with the same result and only recently (after several months of not using it) inserted it in again. I really wanted to keep my machine free from scale and forgot about the bloated feeling. Well a couple of days of drinking my morning coffee's i'm super bloated again. The coincidence is uncanny. I can reproduce the bloating by just using the pouch so I know it's definitely what's causing it.

Am I alone with this or has anyone else experienced this? I have no idea the science behind the pouch and how it softens water (i'm assuming it binds certain minerals to it?) but I do know that if the pouch is in, within a few days the bloating will start. When removed, it will go away. So it's very strange.

The pouch uses sodium to soften the water. Sodium can also cause water retention.

There are two different kinds of in-tank softener pouches, and one of them, as belegnole said, is a conventional softener that replaces the hardness ions in your water with sodium ions. And high dietary sodium can be related to water retention.

But if you do the math, I don't think there is enough sodium here to explain your bloating. Your water there probably has about 110 mg/L total hardness, but for a safe overestimation we can do a calculation based on a total hardness of 200 mg/L (as CaCO3), and also assume that your softening pouch is 100% effective and exchanges every one of those hardness cations with two sodium ions, adding 92 mg/L of sodium ion to your water. In a 40 ml dopio espresso that would be a little less than 4 mg of sodium that the softener adds to your espresso.

A large stalk of celery contains over 10 times that amount of sodium.