Its easy to overtighten toe straps actualy. In the days when we raced with toeclips and straps we also had cleats which prevented foot movement. Without cleats straps alone aren't that solid and it sounds like you have tightened the straps right up to compensate. Loosen them off a little. The quality of the shoe that your wearing (or non cycling specific nature) won't help either. Back in the old days cyclists would tighten the straps for specific efforts and then loosen them off a little when the going was easier.

Regards, Anthony