If you want a doctor to respond you need to post in the expert forum.
I am a bit rusty but lasix is a presribed diuretic? Why are you ingesting salt and caffiene? Go see the doctor that prescribed the medicine!
You should get a doctor to examine your kidneys directly. I suspect this could be kidney stones. It can cause severe pain in exactly the way you describe. Since it is very painful you may need them to be broken up. High frequency sound waves can usually break them up, and a doctor can help with that.
Some symptoms of kidney stones are bleeding while urinating, upset stomach, and trouble with acidic or salty foods (which is exactly as you describe).
I doubt this is anything sexually related.