my gain was 0.031 / 0.032 btc
now it dropped to 0.025btc
it's normal?

Higher pool hash rate = faster block finds = more payouts to be divided by more user hash = lower payouts when your hash rate has not grown at the same percentage like the pool.
So at the end, it does not really matter. The nice thing about the higher pool hash rate is the lower variance (means more regular from a time perspective) in payouts for the individual contributor.
To sum up: Higher pool hash rate = more and more regular but smaller payouts