You have been diagnosed with kidney
stone which is 3 mm in size and I guess they have found no other pathology. Kidney stone explains your pain, nausea
and dark urine which means blood in your urine. The floating tissue in your urine is most likely blood clot.
3 mm stone usually pass spontaneously and only thing you need to do is take good quantity of water and symptomatic treatment for pain. However if pain and dark urine persists you need to consult your urologist
to confirm the status of kidney stone and to rule out other pathology.