W.W.
Welcome to mamapedia!!
You need to go see your OB/GYN or your PCP for a blood test.
I've never had a blood test be wrong.
I've never had a HPT change on me from + to - so I can't advice you other than to call the 800# on the back of the box and ask them what might be going on with their test.
Otherwise? Go to your regular doctor if your OB/GYN is closed.
Good luck!