I'm using pills for family planning, didn't see period december, i bought pregnancy test at the pharmacy & results were negative what cud b the probl?

Minimal endometrium: The use of the oral contraceptive pill can minimize the the length of the endometrium as an additional prevention for potential implantation. As a result, often at the end of the menstrual cycle, there is very little endometrium to shed resulting in a "silent" period. If the PG test is negative simple start the next pack of pills if you did not miss any of the previous doses.

Birth control : It sounds like you are saying that you are taking birth control pills but did not have a period and that your pregnancy test is negative. This is fairly normal on birth control pills. Sometimes people do not have periods on them.