I NEED HELP?

i came on a normal period dec.28,2007...ma period always comes on at the (end) of the month but wen it came on in dec..it ran into da beginning of jan. and like around jan. 6 i had unprotected . so now its feb.. i neva came on ma period for da month of jan. and i missed ma perios b/c ma lil sis and ma mom always come on after me..and dey both came on for da month of jan. but i didnt...wat could dis mean.???