I'm 37 and just had my 2nd miscarriage in my 5 week of pregnancy. what could be causing my miscarriages?

Several things: Unfortunately, miscarriages are common - 20-25% of pregnancies end in miscarriages. This is increased when the mother is > 35 Y.O. I would recommend that you speak with your doctor. Recurrent miscarriages warranting a full workup is recommended after 3 miscarriages. But frequently, there are some initial lab tests that can be done to ensure that you do not have a treatable medical condition.

Miscarriage: Miscarriages early in pregnancy are usually caused by an abnormal chromosomal complement in the fetus. As you get older chromosomal abnormalities become more frequent, that increases your risk of a miscarriage as well as a chromosomally abnormal child, such as down syndrome.