What Causes Frequent Urination?

I had sex with my friend a month ago..no ejaculation tool place...two weeks later she had periods, on the date it was due...now a days she is frequently urinating...when checked on the internet..I found..frequent urination is a sign of pregnancy and Implantation bleeding could be misunderstood as periods..we got worried and got a home pregnancy test done 3 days back..the result was negative...I am worried why is she urinating a lot..could she pe pregnant? what should i do?