What Could Cause my Lower Lip to Turn Purplish?

Thank you for your question. This may just be a venous malformation on your lip area due to the increased vascular load during pregnancy.  See your OBGYN,  a Board Certified Dermatologist, or Board Certified Plastic Surgeon for further evaluation.  This may require laser in the area, but I would first wait a full year and see if it resolves on it s own. I hope this helps.

The purplish discoloration is not cyanosis as would occur with cardiac shunting as you are suggesting, especially if you see veins. It is likely to do venous lakes or malformations and these can be treated with sclerotherapy or long pulse lasers. 

Hello

You sound to have a Hemangioma. The hemangioma got worse or more noticable with pregnancy due to hormonal effect.

You need to consult with a plastic surgeon for proper diagnosis and options for treatment.

An angiogram may be needed to show the extent of the hemangioma and the feeding vessels and the possibility of clotting these feeding vessels to shrink the hemangioma