Mary Janelle, it sounds to me from your post that you need more than what can be answered on this board. If you put a support post in one spot between the first and second floors, the load will have to go somewhere and could cause even bigger problems in the first floor/basement areas. I think you should look at getting a structural engineer involved.
One question with all this. What has happened recently that might have changed the way the load is being supported? Did you put in a waterbed? Have you had plumbing or other work done where someone has changed the structure? Don't know where you are located but has there been unusual weather or a change in drainage pattern that might be causing problems? Any chance of termites or carpenter ants? There are a lot of potential hazards and you will probably need to resolve what is going on. Henry