Start by removing both pedals, cleaning both the threads and most importantly the faces where the pedal meets the crank. Reinstall and tighten tighter than you did before. With luck that's all it'll take. Otherwise, it's probably that you assembled the left pedal or the BB loose.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.