First of all, urine when you feel the urge and don't hold it in! Secondly, you are doing right by not taking any pain med when there is no pain, BUT take your Flomax everyday! Flomax will help you pass the stone.
If you still have blood in your urine, most likely you have not passed your stone yet as your Doctor believes and it was still in your kidney AT THAT POINT. Keep drinking your water to help flush out that stone and also straining when you urinate to see when you pass it. It is not unusual to have severe pain and then go without pain for days or even a week or more and then get hit with the pain again. The pain occurs when the stone is traveling thru the kidney and when it is stationary, you have no pain.
Go back to work if you feel fine and advise your employer of the situation just in case you have another episode and need to get a relief driver if you are in the middle of a run when another attack happens, if it ever does. There is a chance that with a stone of just 3mm, that you could just pass the stone and not have anymore pain associated with it, depending on where the stone is now. Your Dr. can determine where the stone is now with another cat scan on your next visit. Hopefully, it will show the stone has passed into your bladder already. I have never had any pain once a stone that size passes into my bladder. From there, a 3mm stone shouldn't be a problem passing on its own. Good Luck.
Unfortunately, kidney stones are very unpredictable.
Like Unclebuck257 says they cause pain and infections when the travel down the urinary tract. You can go for days, weeks or even months without having any problems when the stone is not moving and when it starts to move you may feel some pain, a lot of pain, pass a lot of blood, feel nausea and have diarrhea, high temperature etc.
As you are feeling fine, you should go back to work, but inform your employer that you have a kidney stone that by the sounds of it has not yet passed. Although the stone is 3mm it can still break down into small fragments as it comes down.
Sometimes kidney stones can get stuck, so if you do encounter lots of pain and feeling very ill, do get urgent medical attention. Ask the doctor to refer you to a urologist if you have not already been referred to one. A urologist may do more imaging tests to find out where the stone is and on its progress and will decide whether it should be left to come out on its own, or whether you need medical help to get the stone out.
Make sure that you keep drinking plenty of water as advised by your doctor and never hold if you have an urge to empty your bladder, you will get more infections doing that. I should image that being a bus driver this is going to be a bit hard to do. Can you not speak with your employer to give you some other duties temporarily?
Do urinate into a clean clear jar so that you can observe your urine. If you notice visible blood, a very strong smell, cloudiness, this will indicate that there is a possible infection and you would need to see your doctor to check your urine and possibly may need a course of antibiotics if the test proved positive. Sometimes you may notice just very fine sediment or grains of sand and this will be part of the stone.
You will know when you pass a stone, because you will feel a lot of discomfort and pain as this passes through your urethra. Sorry.
Now that you know you have a kidney stone, be aware that you will be at a high risk of getting kidney stones again. To avoid this happening, make sure that you drink plenty of fluids and do not get dehydrated.
When the stone does come out, get the doctor or urologist to send it to the laboratory to find out its composition. There are different types of kidney stones and it depends which type you have , will also help you to avoid getting future kidney stones by avoiding certain types of foods.
Hope it comes out soon with the minimal of discomfort.