I suddenly am unable to run. I can only walk VERY slowly.
You are possibly encumbered. This means that the items you are carrying are too heavy. Sometimes this may even prevent you from moving at all.
To resolve this, you may need to seek out an NPC to sell your goods to in order to be able to run again.
If you are not encumbered, it is also possible that you are stuck in world geometry. You can find more information about this here: Stuck in Geometry.