Check this link out if you downloaded the amounts specified during the time periods shown you will have your speed cut to the limits listed.
http://www.virginmedia.com/help/traffic-management.php
But it is more likely to be a problem elsewhere as your speeds are to low as stated by the others above so you will need to call Tech Support is open 24/7 on 151 from your Virgin Media Phone. It's absolutely free.
Or call 0845 454 1111 from any other phone line.