Womens health - periods every 3 weeks???

I am 36 and have been on birth control most of my adult life. 4 years ago I decided to change to the 3 pack birth control - so I got my period every 3 months then 2 years after starting it I came off birth control completely because I wanted to prepare myself to start a family. In any case my periods returned immediately and normal - every 28 days and remained that way for about a year. Now however I am getting my periods regularly every 3 weeks on the dot and I have noticed I am more tired than usual - I tend to be slightly on the anemic side anyway. My concern is that my NP suggested I may be premenopausal, thyroid, or fibroids and my anemia may be worsening. I am waiting for labs on my iron and thyroid levels and I am scheduled for an US. My concern is that I still want to be able to have children and is it possible that I am premenopausal and if so why didn't my NP check hormone levels or could this all be related to stress. Looking for any and all advice and support - thank you Sam