Thin endometrium but normal ovulation

I'm 28, no children, have been with my partner all my life and we've never slept with anyone else before then - we're both very religous. I used a copper coil for 3.5 years. I used to get very heavy periods, before and during the coil, which lasted 5 days. Since taking out the coil last year, I had one miscarriage, during my first cycle, at 5 weeks  (three positive pregnancy tests) - at miscarriage endometrium was 5mm. All my periods are regular as clock work, but since having the coil out they are very light, only lasting about 24 hours.

Hormone tests are normal (FSH, LH, Testosterone, TSH), and day 21 progesterone was normal (i.e. I am ovulating) and ultrasound confirmed a corpus luteum but still a 4mm endometrium!

What is going on? Is there any hope?