Could my wall have been damaged while the painters were painting?
The damaged area could have been the place the shower head pipe may have originally projected out of the wall. When the rain type show head was installed it may have been moved to a lower level. 

If this is the case then the damage that you see is just a shoddy hole repair. Fixing it up so that it looks decent will involve scraping, sanding and repainting the area as a minimum. Worst case it may mean removing the old patch and filling in with a replacement patch that was done correctly. Patch technique will depend somewhat on the type of wall it is. You mentioned plaster - is it on lathe or plaster backer board of some type?