What Is The Cause Of Weakness And Frequent Urination ?

I was wondering if my girlfriend is pregnant? We had intercourse about 4 times on the last day of her period & it seemed her period stopped around the second round. I pulled out on time, each time but did not think of any sperm in my urethra & she does take the pill but, god help her, she isnt really keeping good time with taking it. Her period came right on time as usual, 28 days, but this time it lasted seven days instead of three to four. She has been urinating frequeantly & was fairly fatigued last time I saw her but I think that is irrelevant because this was happening the day after intercourse & I doubt she would show signs so quickly. Also she is 18 yrs old & has not shown any other unusual signs.