help me ladies please..

so my period has been clockwork it always been 26 or the 27th of the month I usually keep a calendar .. now im 6 days late and experience lower back pain and really tired.. I highly doubt Im preggo.. my husband has a vasectomy back in June 2016 after I had my 3rd child.. he always went for the test and he pass he is infertile .. now out of the blue wondering im missing a period. yes I still breastfeed my 14 month old but that has nothing to with it because I still have my period when i breastfeed my baby. .please help me and thanks.