Hi Miramoto

I think it is normal. It is the absolute hashpower of the pool is growing, so your share is going down because its stable. As the time of 1 shift is decreased by the growing hashrate of the pool your relative share is decreasing.
This is nothing to worry about because the growing hashpower of the pool makes it more likely to find a block.
on the other hand increased difficulty with is a result of the growing hashpower of the whole bitcoin community will reduce your share anyway as long as you not add equal groth to your hashpower.
best regards
joerg