Definitely keep your appt with your doc. It can happen.
But also, you might want to call your doc's office and tell them your symptoms to see if the cramping is anything to be worried about. (I don't remember cramping for a month with either of my pregnancies so it seems unusual to me). I have read that with a tubal pregnancy (ectopic pregnancy) there is sometimes bleeding (as in spotting or a partial period) and abdominal pain. So I would raise that question with your doctor. If that ends up being your situation, you should find that out now, rather than wait a month. You might have more options to save your fallopian tube sooner rather than later. I hope that none of this applies to you and don't want to scare you, but it can't hurt to raise the question with your doc to rule it out... and hopefully confirm a normal pregnancy that just didn't show up yet.
You did not say how long ago your "opportunity" for pregnancy happened, but up to 3 weeks, I wouldn't think anything about the HPT reading negative. Beyond 3 weeks though, I would think it is much more accurate.
God Bless!