Endometrial hyperplasia would be the most likely reason.
This is a "precancerous" condition, an overgrowth of the uterine lining that can potentially turn cancerous if it's not taken care of. It takes a long time for this to happen, though, so you've no doubt got plenty of time to take care of it. Endometrial cancer is very rare in women under 40 years of age.
The way endometrial hyperplasia is diagnosed is that they get a sample of your endometrial lining and send it in to a lab.
This sample can be obtained by D&C, or by endometrial biopsy, an office procedure.
If it does turn out you have hyperplasia, they'll put you on either birth control pills or progesterone supplements for three months, after which they'll do another endometrial biopsy to make sure the hyperplasia is gone.
Three months of progesterone usually clears up endometrial hyperplasia, and then you don't have to worry about getting cancer later in life.
If it's not hyperplasia, it could be endometrial polyps. They sometimes distort an ultrasound and make it appear that you have a thickened lining. These polyps are typically benign, and can be removed in an outpatient procedure.
Or maybe it's nothing at all.
I had a 20 mm endometrial lining back in December. My doctor did a D&C, and I was sure I had hyperplasia at the very least. But it turned out my lining was completely normal.
With younger women who still menstruate, a thickened lining doesn't necessarily mean anything. It's mainly a warning sign in postmenopausal women.
But they'll still need to check it, just to be on the safe side.