Gained 12lbs in a month

Just got back from my 28 week OB appt and was told I have gained 12lbs since my last appt. Up until now I have only gained 4lbs this whole pregnancy so I'm wondering what the hell happened. My glucose test came back normal and my BP is also normal. My doc is not happy about my rapid weight gain and wants me to stick to a low carb diet. I've been crying since I got home, I'm so embarrassed and mad at myself😥 time to pick up my running shoes I guess! Sorry for the vent ladies.