ttc - period late - negative tests

hey ladies, im 22, my fiance and i have been ttc for the past 8 months so i have been recording the 1st day of my period and how long it lasts each month. ive been pretty regular,28 day cycle, only varying by a couple of days in length. i was due on the 6th of this month, nothing has happened. know its very early days but i feel different. have had really bad lower back ache, have noticed sore bbs today, have been lethargic for the past couple of weeks. dont usually have any symptoms of impending period!. we had unprotected sex throughout my ovulation time so im wondering?. . . however ive taken 2 first response hpt one on the 4th and one today and they've both come out negative. does anyone have any advice? have a tested too soon? any thoughts would be greatly appreciated! many thanks in advance! xx :-D