It may cause you supply to decrease but any breast milk isngood! :) Have you tried pumping? If your already breastfeeding it would be much cheep er a deep your supply up..
Nothing is wrong with doing both. I did it. But I do recommend you switch to formula during the day and breastfeeding at night just because if the baby hungry while you out during the day you dont always have to pop the boobies out ;) lol