help confused

So I had a period on september15th and lasted tell the 20th well I never got a period in October I was sure I wasn't pregnant cuz me and my husband didn't have sex during my fertial window so I didn't stress it so now my period is still not her and I'm in the middle of November I have a blood test on Wednesday but what do you guys think is going on has this happened to anyone i keep  getting negitive test but end up being pregnant?