Will estrogen dominance always cause uterine cancer?

Estrogen/Uterine CA. Absolutely not. "high" estrogen levels are only one factor in the risk profile for uterine cancer, which also includes obesity, family history, and oral contraceptive history. Even if you possess one or more risk factors, that does not mean that a tumor is certain. Close monitoring and counselling by your gyn is the best course of action.
Not always. Excessive estrogen, whether endogenous (produced by one's own body) or exogenous (taken as supplement) can overstimulate the uterine lining (endometrium) and lead to abnormal bleeding. Excessive estrogen is a risk factor for endometrial or uterine cancer, but not all women with unopposed or excessive estrogen develop uterine cancer.