little hard bumps on a kitchen wall (photo). any explanation?

[Mazonas]

Hi,
I dont understand what are they and how do they appear on my kitchen walls... for nearly 5 years I've been living in this property, and everything used to be fine till now... nothing changed regarding my living.

recently I noticed a couple of little hard bumps on a kitchen wall. a couple month later more of them... to me it seems that they appear instantly (like over a single night) from no signs of anything unusual starting to form.

they are really small and they are hard in texture... and they are quite strongly attached to the wall. however it is not hard to scrape them off. after I scrape them off the stains remains on the wall.

what could it be? there is nobody else in a household, and I have no pets.. no unusual materials being kept in a house. household is always kept clean and tidy.

I know the household next door has damp problems.. but could it be related? cause as far as I imagine there should be a wall cavity in between our walls... I wonder could the damp problems spread from the house next door... causing something to grow on my walls.

another possibility - spiders laying eggs or forming something else causing that? I have noticed quite a few spiders always being in a house..

any ideas?

[toolbox]

Sounds like the nails holding your plasterboard to the studs are slowly creeping out, that's why screws tend to be better the stain is rust from the nail, test with a strong magnet see if it is indeed metal.