To get toxicity symptoms from vitamins and actually experience organ damage you would have to literally take an entire bottle daily for months.
Certain vitamins (like vitamin C) have mild diuretic properties in the body. I suspect that is what is happening. It's nothing to worry about.
What multivitamin are you using?
Most multivitamins on the market are crap. They are marketed to a very naive audience.
The biggest thing to look for when shopping for a multivitamin is what form the minerals are in. Most supplement companies use mineral oxides in their multi vitamin products because they are cheap. The problem with that is they only have a 2-4% absorption rate. Take magnesium or zinc as an example. Most multivitamins will have the forms of 'magnesium oxide and zinc oxide' listed.
What you want are amino acid chelates. You want to see 'magnesium amino acid chelate and zinc amino acid chelate'.
Check ruthless 3d from n2bm.com. It only contains amino acid chelated minerals. It also has higher dosages of B vitamins, vitamin C and vitamin E. It was formulated with the hard training athlete in mind.