I have pubic bone pain and urinary frequency. doctor ordered a transvaginal ultrasound that came back clear. could it be ovarian cancer? (age28, brca)

Unlikely: With your symptoms it does seem to be likely diagnosis.

Probably not: It is very unlikely to have ovarian cancer at 28yo. A negative ultrasound doesn't necessarily rule it out but is a good indicator that the ovaries are ok at this time. If you are brca positive, then you should have an ultrasound and ca125 test every 6 months which may help a little but even better is to remove ovaries once you have completed your family.