During the first half of your menstrual cycle thickening of the endometrium somewhat is normal but if it gets too thick then it can cause health issues.
If your endometrium continues to thicken then it is caused by an excess of estrogen when the lining is overstimulated.
This excess estrogen can then lead to a condition known as Endometrial Hyperplasia.
Endometrial Hyperplasia is a condition where the lining of the uterus which is known as the endometrium becomes too thick.
The condition Endometrial Hyperplasia is not cancerous although it can eventually lead to uterine cancer so always get tested and checked out so you can be treated in time before it does possibly lead to uterine cancer.