No periods since 2nd child

[123hopeABC]

Hello !!
Im new to this so hoping im posting this in the right place.
Iv always had regular periods.
I had my son 3 years ago and my daughter 17 months later... She is now 21 months old. Since having my daughter my periods havent returned. I breastfeed her and stopped at 8 months so I havent been feeding for 13 months. I called the dr yesterday and hes arranging bloods hes hopong June time but with this visus he couldnt guarantee when. He doesnt think iv gone through the menopause because Iv had no symptoms.. but wants to check and test thyroid. He did mention about a thin womb lining?? I feel fine in myself so dont think it could be thyroid?
I was wondering if any one else has had any experience of this. Im worried now as i just thought these things take take but it seems unusual for it to take so long for things to get back to normal.
Im healthy have a good bmi and look after myself...
Anyone else been in the same boat? If so what was it or can anyone hazard a guess? Thank you so much xxxxx