Help! My 4 week old donkey has just stopped nursing. Why? What can we do?

My 4 week old donkey was nursing just fine right from birth until just today we noticed she seemed very tired, lazy, and we haven't seen her nursing at all. We tried to bring her to the mother's udder to nurse but she isn't interested. We also tried milking the mother to bottle feed the foal. The foal would drink a little but again, was not very interested. Any idea what might be happening? Why would the foal not be hungry? Could it be parasites or another disease? What can we do to get her to eat/drink? Please help! We are getting very worried.