When will nausea stop in pregnancy and why does it happen?

Varies. Nausea/vomiting is very common in pregnancy. 80% resolve by the end of the first trimester (14 weeks). A few continue to have symptoms throughout pregnancy. If there is greater than 3kg weight loss or more than 5% drop in pre-pregnant weight, you definately need more aggressive therapy. Ginger extract, vitamin b6, Phenergan (promethazine) and other therapies may help and are safe. Avoid greasy/spicy foods.
3-4 months typical. Nausea in pregnancy is related to hormone levels (nausea is a good sign - more nausea means less likely to miscarry!), and typically lasts until about 14-16 weeks of pregnancy. Severe vomiting is not normal and should see doctor right away. For nausea relief, try plain crackers, ginger ale, ginger, sea-band device for pressure point on wrist, or vitamin b6 (50mg once or twice a day).