Missed Period... HELP!?

So I should have started my period about 2 and a half to three weeks ago, but I never started. A week and a half after my missed period I took a pregnancy test that was negative, the next morning I took another one because the test said that they were more accurate in the morning, but that test came back negative as well. I havent felt much different and I still havent had my period. Is it possible to just skip a month or what else could it be? I am also only 17