What Causes Missed Periods And Vomiting While HPT Is Negative?

Hi I am a 22 year old female. Ive missed my last two menstrual cycles. I have been vomiting throughout the day for the past two weeks or so. Almost everything besides soup like foods, sandwhiches, smoothies comes back up. I eat and drink and it comes back up. Even water and certain juices. I feel really full after only eating only a little. Losing weight but I assume that's due to the vomiting. Ive taken 4 pregnancy tests and they all came up negative. I really miss food. what can I possibly have and how do I fix it?