possible to still be pregnant?...

Gemma
So we have been trying for baby for 6 months and using app to track AF etc. This has ALWAYS come on time on exact day predicted by app. This month no AF and am now 4 days late which has never happened. Have done two pregnancy tests however on day I was due and two days later and both negative. AF still not arrived though. Can I still be pregnant? Has this happened to anyone else? Thanks xxx