pro tools express memory problems

[music@johncain.info]

Anyone know what I can do to solve this problem? I keep getting a message upon opening a pro tools session that says "pro tools is critically low on memory. close your session and reopen." Then pro tools will shut itself off. Everything has been working fine till now. I can open previous sessions (that have lots of data and tracks etc.) and they still work fine, but I can't start a new session without it shutting down. I went through and cleared trash, and moved all pro tools sessions to an external hard drive.