Thanks for writing to us with your health concern.
As you are having two negative tests at home, the chances are very remote that you are actually pregnant.
Also, since you are menstruating regularly , and hence ovulating on time, it is vital that you should use proper contraceptive methods now.
Sometimes, a period might be delayed due to anxiety, stress, weight gain, ingestion of certain medications, travelling etc.
Mild derangements of the thyroid hormone, polycystic ovaries or endometrial hyperplasia - these are conditions which can cause delayed menstrual cycles.
As this is the first time this has happened to you, I would ask you not to worry at all.
Keep using some contraceptive though ( condoms till you have your next period and can choose birth control methods ) .
If you fail to have your periods for a week more, then consult a Gynecologist to rule out the other conditions ( apart from pregnancy ) as causes.
Do not stress out, as stress itself causes delayed cycles.
All the best.
Please feel free to ask for further clarifications.