What Causes Delayed Period And Weight Gain While HPT Is Negative?

Hi, I have not had my period since september 1st. thats 5 months!.. I been very tired alot. I have been recently getting severly car sick on 10 min car rides and very motion sick just playing on my cell phone!.. I have had wierd cravings and cramps and weight gain I feel like im going to throw up alot and a few other symptoms.. I have taken a hpt and all 3 came out negative.... whats going on????