You may want to check your VAC status (Valve Anti-Cheat).
1) Please launch the Steam application from your desktop and login.
2) Once Steam has updated select "Steam" from the upper left hand corner
3) Select "Settings" and just under the "Account" tab displays
**Account Name - yourname
**Contact Email - email@example.com | Verified
**VAC Status - In Good Standing
**Security Status - Protected by Steam Guard
If your VAC Status is anything other than "In Good Standing", then the issue is with Steam and the VAC system.