Possibly Pregnant!!

Ok, so last period I had was April 20th-24th or 25th. I was "supposed" to get my period May 18th (my periods had always been irregular and I've been known to skip a period sometimes) but never got it. I had sex May 17th for the first time in over a year (protected, comdom never bust or anything like that). I hadn't had any pregnancy symptoms and I've taken three tests, all which have been negative. I even took a Clear Blue early detection tests and it still said not pregnant. I'm about 2 and a half weeks late from when I was supposed to get my period... I've also been stressed a lot and I've been staying up as late as 4 in the morning and I sometimes barley eat and when I do eat, it's once or twice a day. I've been trying to get myself on a good schedule but it's hard with the job I have.... My mother and I also think I may have a hormone problem as well.    Am I over thinking things or could I actually be pregnant?