False negatives ?

So I've heard of false negatives and false positives but never looked into them and if they can actually happen. But I haven't gotten my period in 17 days now. I was spotting a while back and thought I was getting my cycle but I never did. I've tested about three times (all different time period/days) and all were pretty faint, but all three said Negative. I know you can miss your period from stress but I havent felt stressed in a while. Could I still be pregnant? And yes we are TTC.