As the pool grows and the hash rate increases and assuming your hash rate stays the same, your rewards will be less BUT the pool is expected to find more blocks so you will get more smaller payments. As kano confirmed, yes your payments are normal and expected.

FYI my rewards are now about 1/4 of what they were 2 months ago, but then we were only getting 1-2 blocks a day where as now we get around 3-5 blocks. (I haven't worked out the average number of daily blocks)


EDIT: I meant 2 months not 3.