Very often, when a woman has just been started on HRT or in your case, Bioidentical hormones, bleeding can be common. However, it is important that your doctor rules out any possibilities. If you had just started bioidentical hormones, then this is likely the reason why your doctor simply said "It's an Estrogen thing". However, now that you have gone to your second doctor who prescribed Progesterone, please make sure you provide this information with the surgeon who does your insertions.
While bleeding is most commonly related to the shifting in hormones, if you truly feel like you would like further testing to rule-out hyperplasia of the uterus, you are your own advocate and you can certainly ask for further testing to be done.
Here's a list of our menopause groups-- browse through them you may find one of more of them to be helpful: https://www.empowher.com/condition/menopause/groups