Uterine lining only 5mm - what can I do?

12dpo today, af supposed to show but nope. Having major stomach and boob pains, and temp rise but all BFNs. Doc did ultrasound today to see why I'm in so much pain but said it's unlikely I'm pregnant and uterus lining is *only 5mm.*
She claimed my pains were ovulation related and I only just Od 2 days ago based on how thin my lining is. 
I'm so confused. I chart and use OPKs which indicated I Od 12 days ago, same as every month (I'm quite regular). I'm inclined to think she's wrong and actually, I just have a problem with my lining. Is there anything I can take, herbal or otherwise, to help improve it for next time? Thanks