That's what I was thinking too.

It's fairly common for the left crank arm to loosen and even to completely fall off. When that happens it means that the square hole in the aluminum crank arm has been boogered and the arm will never stay tight after that. The solution is to replace the crankarm and the problem is common enough that better bike shops keep generic left crank arms in stock.

Based on the information that you provided, I suspect that's what's happening to your bike.