Unprotected sex while on birth control

i was on the depo shot and i was suppose to get it again in august but i was in jail. So when i had got out of jail i called the doctors n had asked them when i could get back on the shot again n they had replied when i get my period again. It has been 4 months n i still have not gotten my period i been takin test but they are comin up negitive could i still be pregnant or is this normal not to have your period for that long after bein off the depo??? Does stress matter also could that be a reason why im not gettin it? I was also having unprotected sex while on the shot before n after the shot for about 2 years now and was only one the shot for 3 months.