What Causes Nausea While Pregnancy?

I am 15 weeks pregnant, not really had the morning sickness, just had nausea. that passed about 4 weeks ago and ive been very normal since. today when i woke up, my arms were shaking and my legs were like jelly. my heart was pounding and i had to sit down immediately. i have eaten my breakfast and drank some water but i vomited about 30 mins later. just need putting at ease really :-)