K.S.
Go out and get another cheap pregnancy test - You'll feel better.
Chances are slim that you are. If you took a p-test when you were already 3-4 days late and it was negative...Then it was most likely correct. But I can tell you that the more you stress about your delinquent period...The longer it will delay.
So why are you "so late"? Probably not what you want to hear - But 35 is this super awesomely magical age where all sorts of lovely thing begin happening. For instance, did you know that at age 35 (statistically speaking):
You can no longer build new bone mass
A pregnancy is called "geriatric pregnancy"
And so a changing period as you age is to be expected.