Why is my laptop blocking after closing and opening up my screen 3 times ?

I have a Toshiba laptop with windows 7. Due to my job, I have to quickly interrupt my laptop session and close the screen several times a day. When I come back, and open up the screen, my laptop went in log off mode, and I have to push the power on button to get my latest session again. The problem is that after several times (the third time of so), my laptop blocks and I have to do a complete reboot. Is this normal ? What can I do to avoid that phenomenon ?