Out of memory?

I flew on some event in VATSIM,extremely busy,around 20 planes in the air holding around the airport(EGLL)when I started my approach to the airport,i received the following message:"Your computer has run out of available memory. Flight Simulator will now exit.You may not have enough free space on your harddrive. Run Disk Cleanup to free up hard disk space, and try running Flight Simulator again."I had the Task Manager open on the other monitor and the data was that I had 1400Mb of memory in use from 2024 MB.and the CPU was around 40-50%(E6750).Concerning the disk space,In the partition that the simulator is installed I have 149GB free,on the windows partition I have 72GB free,so the disk space isn't a problem.I used Aerosoft EGLL 1.1 and PMDG744 with Activesky and FSINN.Anyone maybe know what the problem is?Thanks.