Are you still having a period within those 4 months? If not, you need to see the doctor whether your pregnant or not. It is Possible that you could be preg with negative results I suppose, but it's unlikely. I suggest if you are that worried that you go to the Dr and see whats really going on. If you have been having your period these 4 months, then most certainly not pregnant.
Also, another reason to see a Dr is if you are that worried about getting pregnant you can talk about birth control options. I know what its like to be paranoid about these things, but at least if you aren't wanting to become pregnant you can weigh your options.
Good luck!