Very interesting. The same thing happened to me on April 13.
On April 12 I served 20,000 pages and used 64 mb of memory (where it had been for weeks) on my VPS. Then on April 13 I served only 15,000 pages, but memory shot up to 142 mb daily average (where it remains), peaking near 300 mb.
I have a WordPress memory plug-in that tells me that the memory used to generate the pages hasn't changed.
What I observe is that I have sometimes 10 PHP5 processes in memory with NONE of them actually running.