A positive OPK doesn't guarantee ovulation, it only tells you that you had a sufficient amount of estrogen in your body to have created a follicle large enough to trigger a LH surge.
When ovulation happens, the egg is released and the empty follicle starts to produce progesterone which makes a significant contribution to the thickness of the lining and the amount of blood in your period.
If you only have a bit of blood, then you are right that your lining must be thin, but it also means that you aren't getting anywhere near enough progesterone which leads to to assumption that you did not release an egg resulting in the empty follicle.
They should be able to see the empty follicle of a scan about 7 days before AF too xx