What Causes Missing Periods, Nausea And Weight Gain?

I'm 3 months late with my period, have been feeling nauseas in the mornings and throughout the day but have had many (upwards of 20) urine tests come back negitive since my last period in March, had negitive urine tests in the doctors office along with a negitive blood test in the doctors office. I have also gained about 30 pounds in the last 6 months, but I beleive that has came from changes in activity level at work. My co worker has mentioned the fact that I may have a Thyroid problem... any ideas?