Hi I am 17 and just come out of a relationship with unprotected sex and I missed my last contreceptive injection and I have had all the symptoms of being pregnant but all the tests show up negative can anyone explain?

Pregnancy symptoms can mimic many other symptoms. Unfortunately the symptoms are never black and white and almost never "by the textbook". The symptoms could be because you are about to start your period or could be your body reacting to missing your last contraceptive injection or could be something else. Sometimes you just have to wait and see what your body does.
However, if you did have unprotected sex there is always a chance of being pregnant. Some women don't get a positive pregnancy test right away. J_9 said she didn't get a positive hpt until she was 4 months along. If you have any doubts make an appointment with you doctor. You can get a blood test done, which is more accurate than an hpt.