Basement I beam set too high
You need to conduct a careful, thorough level survey of your house. Using a rope is completely inadequate. Even a stringline is not too useful for a settlement survey. There are three tools that work very well for an accurate elevation survey of a house.
The most common is a builders level, which can be purchased for somewhere around $500, or rented from most any rental store. You may need some instructions on how to correctly use the level, but they work very well.
An alternative is a laser level, which can be fixed or rotating. I picked up a pretty good one for under $100 at a big box store, they work nicely, and they are easy to use.
A third option is a fluid level, which can be purchased for typically under $500, or rented at some big box stores.
The objective is to make accurate measurements throughout the basement, around the foundation both inside and out, and on the first floor. Measuring up from the basement tells you nothing, as has been pointed out the basement may be out of level.
Once you have performed a careful survey of the whole house, you will be able to determine if the house is settling, and if so whether it is settling unevenly. You can also determine if the beam is level, and if not, how far out of level. A steel beam deflecting as much as you suggest would be catastrophic, and it seems unlikely. My guess is the rope technique was not precise.