What Causes Delayed Periods Post Unprotected Sex?

I am 25 years old and currently in a hyper syste. I had a unprotected sex about 3 weeks back and since then I have missed my period. I didn t get any symptoms of pregnancy so far but I did feel like vomitting for two days which is also because I have UTI. Recently I tried to check if I am pregnant by a test kit,one in the evening which showed negative and the other in morning Postive result. I still have UTI. Please help me.