Occasional pain after urination. Doctors took urine sample but couldnt find anything.

Patient: Hello, im 19 years old male. My problem is that sometimes after urinating i get aching pain in my lower stomach. the pain is not instant- when i pee it doesnt hurt but in something like 3-5 minutes it starts to ache (not a razor sharp pain) and it gets even worse if i try to strain my stomach muscles or move a lot. And when i pee i can already tell if it will ache because when im almost done peeing the urine becomes very cloudy and even with particles. Now the weird thing is this happens to me occasionally- the intervals between painful peeing sometimes are from 2 weeks to 3 months. And when it happens it ache for like 10-20 minutes and then fade away. The next urination is just fine and without pain. The one more thing i noticed that it always happens to me at evenings- never at mornings. I went to the doctors and they took urine sample (ofcourse it wasnt the one with cloudy ending and pain) and they couldnt find anything what got me even more frustrated. Im afraid that this could lead to serious health problems. What i can do?thank you