One of the more frequently asked questions recently relates to nausea and even vomiting when breastfeeding. It may be due to norovirus, pregnancy (see https://breastfeeding-and-medication.co.uk/thoughts/suffering-from-morning-sickness-and-still-breastfeeding ) or to stop nausea from medication e.g morphine.
I hope this factsheet provides some options. The information is taken from my book Breastfeeding and Medication which provides more details and references. Maybe you need to buy a copy?