What Causes Excessive Weight Gain And Missed Periods?

I am a 20 year old female height is 5'4. my normal weight is 120 but i have gained about ten pounds in less than a month. Havent had a period for three months took about 5 hpt and one blood test all are negative. At the end of lest year i got off the depo shot BUT my periods where back to normal until now which have had no kinda of bleeding for 3 months. My stomach is bloated but bloats even more so after i eat, i have irregular bowel movements moody just an oer all more protective of myself like an instinct. please help