As a doctor, do you think i'm pregnant if i got a positive test but i have a 5 cm ovarian cyst, & no gestational sac on ultrasound??

It is very possible: A positive pregnancy test without a noted gestational sac can occur in very early pregnancies. The ovarian cyst could also be a corpus luteum cyst of pregnancy.

Yes: the pregnancy test detects human chorionic gonadotropin (HCG), which is produced by the trophoblast, which later becomes the placenta. An ovarian cyst will not produce HCG. If you are 3 to 5 weeks, the gestational sac is easily missed. Your OB will address the possiblility of a tubal pregnancy, and can confirm that you are not having a miscarriage by documenting a rising HCG level or repeat US.