Welcome Michelle:
I agree with InspectorD, you need a complete home inspection. There is a possibility of other damage that is not so appearant.
If we just look at the date of construction; the norm at that time was to build on piers and sills. Your pictures show a sinking pier or a weakening sill which could be jacked up and repaired fairly simply. It also looks like there may be a heat duct running through the room which could affect the plaster some.
The sagging is definately structural damage, please have it checked out.
Glenn