It could be that she is still not feeling well enough or it is still uncomfortable for her to nurse. Continue to try while ensuring that she is getting enough food and fluid during this period. At the same time, pump to maintain your supply. You can refer to this article with tips on how to get a baby back to breastfeeding:
http://kellymom.com/bf/concerns/child/back-to-breast/
With regard to her pulling off before letdown, could it be that she is just impatient and got distracted? Since she is still able to latch on at night, it could be that she was simply getting distracted during the afternoon feeding. You can try nursing her in a quiet place in the afternoon to avoid distractions. You can also try different nursing positions such as having your baby standing up, sitting on your lap while facing you to keep her interested. Nursing more frequently without her asking may also help.
Regardless, hope your baby gets well soon! Monitor her condition for a couple more days and try some of the suggestions to help get her back onto nursing. Take care of yourself too!