Hi there and welcome to JM! I am sorry you are going through a confusing time right now.
I think one of the most confusing things is how similar pregnancy symptoms and signs that your period is coming are. 28 days is very late though - I would think even if you are not pregnant your doctor might have some concerns or answers as to why you are so late. I have never heard of someone being pregnant but having a negative blood test - but I am no expert.
There is another board on here -
Am I Pregnant - JustMommies Message Boards I think you will find more people to help you out in there.