Might be too soon to tell with pregnancy. Some don't get a positive pregnancy test until 5 weeks along. I would retest in 1 week and see from there. If not pregnant, you might want to pay a visit to your gyn to make sure it's not a cyst or anything.
it could be that for some reason you ovulated late this cycle, which means your period is going to be late. it happens once in a while. the length of your luteal phase (the last half of your cycle after ovualtion) is almost always the same, but sometimes it takes longer during the first half of your period to get to the point at which you ovulate and only after you ovulate do you go into your set luteal phase. so i am guessing that you are just late for whatever fluke of a reason. wait a week. still no period test again. it could be too early for that to be right, too, if you ovulated late.
A lot of girls can have late or early periods. It could be from anything. You just have to wait and see and not panic for now. Deal with it as it comes. It may be to soon to do the tests, I am not sure but I think you have to wait longer to take them. This happened to a friend of mine, she was late for about a week and really thought she was pregnant, took three tests, she ended up getting her period in two days.