What are these spots appearing along seams? When I paint on this texture and save, it kind of resets at the seams and gets these strange parts (didn't happen with another texture):





The .jpg that is saved contains these strange little grainy textures at the seams again.
Also when I paint at these spots with the draw brush and save, it kind of makes the new drawn color transparent over the grainy texture and shows it up again.
It is only at the seam, tried to work around it but I don't have a clue what causes it. 

I guess this might have happened to someone else and probably asked before, so I apologize for a possible double question.

Any suggestions?
These look like jpg compression artifacts. jpg is a lossy format, which means that the saved image is not exactly what it was originally (the difference depends on the quality setting the file was saved with).

The advantage to lossy compression is that jpg files are typically very small compared to lossless formats. However it's not the best format to use during production, especially when a file is being edited and re-saved a lot. When saved in a lossy format repeatedly, the image will gradually lose quality. This process is called generation loss.

Try saving to a lossless format like png instead.