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

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.
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.