recent lagging problems

ok so garrysmod runs great on my comp, but today i downloaded the dystopia mod, a few maps, and the mckay stargate addon, and when i went to play theres a huge lag, and in the top right corner theres the pink and black checkered pattern.

I have no idea how to fix it, i removed all the addons but its stil going on

HELP PLEASE

Have you tried deleting your garrysmod folder? Or uninstalling Garrys Mod? Firstly, make sure you have definitely deleted the addon and if it persists, reinstall.