Reproduction system is acting strange.

I always thought I was a fertile person, I mean by pregnant pretty quick, always seems it takes one time and boom, I'm pregnant..... I have 4 kids... 1 was adopted and 3 of my own. I had my last child a year go. I haven't been on birth control since I gave birth. And I have my periods regularly every month around the same time. My husband and I don't mind having more kids or even one more...but i haven't gotten pregnant yet. I've been having sex everyday on my "fertile" days and still haven't gotten pregnant. Just thought it was strange... when I wasn't trying I end up pregnant, when I was on a few birth control I still end up pregnant... but when I am not on anything for a year now and trying sometimes... I am not getting pregnant.

Is my body telling me... look you're done producing... or something may be wrong with me and I should go get it checked out.... I am only 28 years old happily married for 6 years...