Negative Test but still NO PERIOD!!

Aight so i got my period in february on the 17th. I know that there is only 28 days in february but i was thinking that i would get my period on the 17 or 18. I didnt get it, so i took a test and it was negative. So i thought maybe it was to early to test. So i decided i would wait till today 22 and see. I used First Response test and it still came up negative but no period. Can you tell me something. I dont understand why i havent gotten my period and its still saying negative. I am thinking about just going to the doctors tomorrow and getting a blood test. If i dont go to the doctors tomorrow. I was going to wait till Friday the 27 and test again. If it still says negative then i am def going to the doc. If you have any answer to this question. Can you please help me. Thanks for all you help

Crystal