That's always scary. But first lets keep into perspective that you are very young making you less likely to be the demographic of those who get ovarian cancer. Why were they checking you with the transvaginal ultrasound? Were you having some kind of symptoms that needed investigation?
A hemorrhagic cyst is a complex cyst that has blood vessels involved. They aren't that uncommon for women before menopause and rarely have an impact on fertility later on or anything like that. That he sees that as a possibility makes sense but I'm sure that is something you'd like confirmed. They do often pass on their own, so again, it is not an unreasonable assumption that this is what it is.
Did he suggest any follow up? If not, that's actually a good sign. Ruling out cancer trumps getting to uni on time to a medical professional. If he worried about cancer, he'd insist on further investigation before you go. But if he is a shoddy doctor and that isn't the case, I guess I'd like to know. https://www.mayoclinic.org/diseases-conditions/ovarian-cancer/diagnosis-treatment/drc-20375946 This gives ways ovarian cancer is diagnosed. You could do a blood test called a CA 125 test. It's not used for true diagnosis but gives your doctor a clue. And some more imaging tests to get a better view would make sense.
Again, I do doubt it is cancer but know that I'd want to know for sure too. hugs
Most ovarian cysts are not cancerous. Have you had an ultrasound of the same ovary before and it looked exactly like the other one? That would rule out a dermoid cyst, which would have been there from birth and so must have been seen if you had ever had an ultrasound prior to this one. There is also a kind of cyst that contains apparent endometrial tissue (can't remember if it is real migrated tissue or just something that looks like it). Those are called a "chocolate cyst." Point is, there are lots of kinds of ovarian cysts that aren't dangerous, just odd. If your doctor took a watch-and-wait approach, I'd try to relax. If you aren't a virgin, one way an ovary can bleed is to get bonked during sex by the guy's penis. If you happen to have a boyfriend with a penis lengthy enough to do that or a pounding style, possibly that happened and caused bleeding in the past. (A needle biopsy would be able to drain it if it is still fluid.)
As specialmom says, let your doctor's concern level be your guide. If the doc is not concerned, try not to worry. Let us know how it resolves.