What Causes Delayed Period Besides Pregnancy?

Hello, I have been taking birth control pills but I read that you are supposed to take them around the same time every day. I have been taking them every day but not at the same time everyday. I never miss a period and usually you can set your watch to it. It s been over a week since I was supposed to get it and I have been feeling sick all the time. I am finding that I am tired a lot during the day when I usually have energy. I have also been bloated. I took a pregnancy test a few days ago but it was negative. what else could be wrong with me if I am not pregnant?