Freaking out

Okay, so I was supposed to start my period Sunday. Well it is now Tuesday (obviously) and it still has not started. I have not had any sex for the last month. One of my friends and I were messing around a couple of weeks ago, but no sex. I have been stressed because of school. I work in a building with all ladies, so I didn't know if all the hormones would make a difference. Is it necessary to go see a doctor? Or what the heck should I do because I am literally freaking out. I also have been having some weight gain and weight loss. I don't know what else to do. Help me please!