If your tubes are tied and your tests are negative, you probably just missed your periods for another reason- could be anything- stress, diet, exercise. If it continues, though with you not having a period, I would go into the doctor's office just to make sure everything else is working ok.
It is possible to be pregnant with your tubes tied. I personally know of two women it has happened to. One is our assistants pastors wife at our church. She had three and after that had her tubes tied and about 18 months later she was pregnant again. Then a very close friend of mine had a chemical pregnancy with her tubes tied as well. The risk is that they baby could grow in one of the tubes instead of making all the way down to the uterus. I would definetly try to get into a dr and just make sure. If a baby is in the tube then it sometimes makes it harder for a HPT to pick up the HCG. I would a least ask if they can do a blood serun pregnancy test and that should tell you for sure. I hope this is helpful.