Skimmer Over Flow

[MarkoD]

I just checked my skimmer this morning and it overflowed. skimmer cup was full of water, and entire sump was filled with white foamy bubbles on the surface.

literally looks like bubble bath.

nothing has been dosed in the past few days except Kalk through the ATO

checked it last night before bed and it was doing fine.

anyone know what causes a skimmer to go randomly crazy?