In the first couple of hours until about 1 am (my normal bedtime), I didn't feel very well. I felt dizzy and felt like my heart is beating stronger. But this may well be a nocebo effect because Hip also mentioned getting dizzy above, or the dose was perhaps a bit high.
When I first learned about taking baking soda to reduce urine acidity I tried 1/2 teaspoon on a couple of days and noticed breathing difficulty and symptoms of alkalosis. I lowered the dose to 1/8-1/4 teaspoon a couple of times per day for a week then tried 1/2 teaspoon again without encountering the same symptoms. Now I take 1/2 teaspoon at least twice per day without any problems. My body needed a short adjustment period.
My urine usually has PH 5-6
Below 6 and you're probably at risk of kidney damage from acidic urine eroding kidney tissue.
I also noted that despite drinking 1.5 litres with the soda, my urine was pretty dark yellow.
Your kidneys might be working (filtering) better due to less strain from your usually acidic urine. Another way to see this effect (for me, and lab rats according to one study) is to use a heating pad over the kidneys (especially a far-infrared pad) which causes increased blood flow which leads to increased urine filtration and, often, darker urine.
And indeed I think I am feeling a bit better today than yesterday and lower back pain is a bit better, too, but this may very well be a placebo effect.
If you often have lower back pain then it might be aching kidneys due to your acidic urine.
Excess protein makes my urine more acidic. I found recently that I was getting kidney discomfort and excessively foamy urine (a sign of protein discharge through possibly damaged kidneys) if I ate more than a small amount of protein (even 5-10 grams). I thought it might be kidney failure but then discovered that if I take digestive enzymes (plus my usual betaine HCL) that I can tolerate more protein without kidney ache or increased urine acidity and foam.
I think my pancreas isn't outputting enough digestive enzymes so undigested proteins are getting into my bloodstream and my kidneys are having to deal with them leading to acidic urine, foam (sits in the bowl for longer than 20-30 seconds), kidney ache and probably kidney damage.