Late but negative!

Okay so I'm a week late officially according to my chart, so I took another pregnancy test when I woke up this morning and it was negative. No we aren't trying and I'm hoping it is in fact negative for sure (but of course if turns out positive layer I'd be just as happy) but has anyone else had this issue? It's happened to me one other time since I had my son who is now 16 months old.