What causes gestational diabetes? I'm 20 weeks pregnant and eat healthy, but do enjoy a "splurge" from time to time.

Gestational diabetes. Gestational diabetes is lack of sufficient Insulin to control blood sugars during pregnancy. "splurging" does not cause it.
Bad diet . Gestational diabetes is the result of excessive sugar and carbohydrate intake which causes high blood sugar levels, which then make your Insulin levels rise. Over time the chronically high Insulin levels lead to Insulin resistance and you will need to make higher and higher levels of insulin. You must stop splurging! or you will end up with dm.