memory size error from time to time

Hello,

I have a datatable editor php script used for server side processing. Sometimes, especially the first time requested by the front end, it reports "Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in ......" But after refreshing the page, it successfully responds with json data which is just around 15 kb.

Does anyone have any idea why memory size error reports the first time requested, and then works well?

Thanks.