I know what you mean- I have experienced the same for a number of years now. I am really sorry but can't remember exactly what my fertility specialist told me what caused it, but I know with me, it is due to hormone issues. I last saw her when I was trying to conceive my 13 month old daughter so the memory has gone! But it was a hormone thing. If I were you I would get a referral to a fert specialist from your GP. GPs are not always aware of fert issues and hormanal imbalances. I messed around for years trying to get answers and shifted from pillar to post before becoming quite demanding in my requests!