Super low estradiol level after embryo transfer caused miscarriage?

Not exactly. Actually low estrogen levels after an embryo transfer may indicate that the transfer was not successful and/or if you did get pregnant it might not be a "good" pregnancy and that you are at increased risk of miscarriage. In a good pregnancy, the body produces high levels of estrogen. The low estrogen levels were not the cause of the miscarriage but a symptom of it.