Hi Bibby
This exactly what happened to me. I had two scans and a hysteroscopy, and they concluded that it was endometrial atrophy, as my lining was so thin. Apparently this was hormonal, and is basically when the balance between oestrogen and progesterone is not quite right.
You should of course get it checked out, but it's very common, so please try not to panic too much ( easier said than done, o know)
Jeepers xx