Sudden bumps on wall

Hi,

I have just noticed this appear on my living wall room, it was perfectly smooth before. Does anyone know what the cause could be and how it was would be fixed? It is an outside wall and I live on the ground floor of a three story block of flats.
Any help in pointing me in the right dorecdire would be great. Thanks.

 

Damp probably.

 

Probably is damp but I suspect you want to know why it is damp

 

Just looks like the walls been chased and then filled back in, are you sure it's just appeared and not been there all along?

 

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