Yes you can still be pregnant. Some people do not show up positive on pregnancy tests right away of some reason or other. I would say since you are now a week late it's time to call your doctor. They can do a blood test and some other testing to make sure it's not a hormonal problem or your thyroid that is causing your period to be late. Also if you are stressed out right now, have been exercising, or had a weight loss/gain that could also cause your period to be irregular. Good luck and I hope you get your answer soon.
Get a blood test done by a doctor, that way you'll know for sure. If that comes back negative, it could be a number of things and a doctor will be able to help you work out what's caused it. Best of luck x
I am sure you can find stories (online and among your friends) of someone they have heard of who had a negative test and turned out to be pregnant. However, if you have taken a couple of hpt's and followed the instructions correctly (especially, you used first-morning urine so it was concentrated enough to register), you are probably not pregnant.