What Causes Delay In Periods Despite Having Negative UPT?

I did not have my periods since 2 months. My pregnancy test is negative , doc gave me deveiry and even after a week I have no periods. I have beem diagonesd with bacteria in my urine and stool amd also occult blood in stool. What is the problem. Can I become pregmant. I have had unprotected sex for 2 months now..but no effect