Sweden gets a mission which gives them claims up and down the Baltic coast, don't they? As Prussia, you probably own that land, so its a reaction to Sweden suddenly laying claim to all of it at once maybe?
If thats the case it doesn't show in the province screens. Like I said I checked my baltic provinces and only one showed an (very old) swedish claim.
I think after work today I'll just start the game as sweden. Maybe from their point of view it shows something...