I'd say dehydration if the urine test came back negative for infection. Whatever you are eliminating is so concentrated in your urine that the smell is more powerful. Your urine should be very pale yellow. If it's bright yellow or dark, you're dehydrated. Also, certain foods can change the color & odor - asparagus is one but there are others. That is harmless but it also shouldn't be all the time - once it's eliminated, it's gone. And that doesn't seem to be happening to you. The other possibility is some of the increased mucus from your sinuses & post-nasal drip. The solution for all is more water.
Try cranberry juice as it is a fruit juice that helps the bladder too. I agree that the diet soda is not helping at all and is probably hurting you. You should increase the water but don't overdo it. 8 glasses a day is okay but you may have heard about these crazy contests that force people to drink too much water - at least one woman died. I'm talking about gallons and gallons, mind you, but that excessive water drinking is hazardous. The leaking when you cough is normal. If you rinse your underwear in the sink and then put it in the laundry room to wash fully later on, the smell won't be as obvious and the urine won't stain.