Some women simply don't test positive even when they are pregnant. I've talked to a few women who NEVER tested positive at all during their pregnancy and went full term.
Go see your doctor and let him/her know up front your home tests have been negative. They can do bloodwork and may even order an ultrasound to figure out what is up. Also, even if you aren't pregnant, a late period doesn't necessarily mean something is wrong. Many things can cause it and most are temporary.
Again, talk to your doctor :)
You can go to the doctor and ask them to do a blood test and you will know for sure.