Estradiol Levels

Hi everyone, I had my IUI procedure 1 week ago and just went for my 7 day post IUI bloodwork. My progesterone levels looks good but my Estradiol (Estrogen) levels were low and they want me to take Estrace. Has this happened to anyone else? Is this an indication that my IUI failed or has anyone had a successful IUI but had low Estradiol levels? Any advice/information is appreciated!