Did something happen during the pregnancy to cause my baby to have polycythemia?

Perhaps: Polycythemia means that there are more than the usual number of red blood cells in the circulation. A newborn may be polycythemic for many reasons, the most common being maternal diabetes. Other conditions during pregnancy can cause polycythemia, such as placental insufficiency, or it can be caused at delivery, by delayed cord clamping or low positioning of the newborn before the cord is clamped.

Hard to tell: Certainly polycythemia can be a complication of mom's health such as diabetes, but it can also just happen because a baby gets a little dehydrated, this usually not a serious problem.