Totally normal and not much to be done about it. It's just lint that's accumulated in the corners... Lint coming off the surface layers of the diaper get washed or dried away, but lint from the layers inside have nowhere to go so they eventually just accumulate and form balls. Sometimes if the lint stays spread out you won't feel it or maybe the fabric is thick enough and the balls small enough that you can't feel them much. Sometimes it accumulates in areas where you just don't notice it but it's pretty common.

It's pretty harmless and certainly doesn't mean that your diapers are finished. If it really bugs you, it's always possible to open up the diaper, take out the lint balls and then close it up again.

Karen.