You are at the age where the menstrual cycle is unpredictable. Basically anything goes - no period for months on end followed by heavy and/or prolonged bleeding, periods one right after the other, etc. If it is infringing on your life too much, you can use hormonal (birth control pills) or non-hormonal (tranexamic acid / Lysteda) to help normalize it.
If your ovary(ies) had some abnormality, I would think that the TVUS would have seen it. Although your grandma had ovarian cancer at age 80, if you don't have other close relatives who have had it, you may not be at increased risk. Our risk for it increases as we age but a woman's lifetime risk is VERY low at less than 2% (1.3% to be exact based on govt. statistics). My aunt also had OC late in life (in her 80's). It seems her oncologist was not concerned about her daughters havingt an increased risk. Of course, if this is a concern you could get tested.
I understand... the "worst case" thoughts can really do a number on us! They did for me when I was dealing with a gyn issue. Hang in there!