Hopefully you didn't ride it far enough that the crankarm is already hosed. Loose left crankarms with square taper bottom brackets is a common problem. The torque spec for crankarms is usually around 30 to 35 lb/ft. If you are tightening it with a 6" long allen wrench, you're not getting to 30 lb/ft.
If the crank bolt isn't tight enough, the crankarm will eventually work itself loose and may even come completely off. Once that happens the square hole in the crankarm is usually hosed and the only adequate fix is to replace the crankarm. That happens often enough that every bike shop that I've ever worked in kept a stock of replacements.