What Causes Missed Periods With Negative HPTs?

I haven't had my period in three months. I have taken 4 at home first response pregnancy test and all came out negative. I'm sleepy all the time and now lately I have been getting really dizzy. i have weird mood swings. and I feel like I keep gaining weight even though ive been watching what i eat and making my self be more active. with my weird mood swings i also feel really depressed and its not like me to hate myself.

Do you know what is wrong with me?