I administered a urine pregnancy test 36 days after sex but the urine was dilute (1 hour hold, light yellow in color). girlfriend doesn't have periods and can't use fmu. can we trust the result (which was negative)? if not, how about a week from now?

Urine pregnancy tests are expected to turn positive two to three weeks after potential exposure. Five weeks out you would expect a positive test in even dilute specimens. In another week it is even more reliable. False negatives occur more often than false positive, but this far out I think your results are true negative.

If you have any concerns, get a blood test for HCG levels and that will settle the issue. If you do not wish to be pregnant, use contraception all the time, every time. Consider implanted contraceptive, or IUD.