Sometimes during the evening i get like little electric nerve cramps in my lower back and legs it's not painful but annoying is it from being tired?

Hard to say...: Depends on if you mean actual cramping or aching in the back and legs, or tingling and shooting electricity feeling. Aching could be from being tired or from overuse (heavy lifting). Tingling could mean an irritated nerve in the back. Sometimes, restless leg syndrome causes an annoying discomfort that makes you feel like you need to keep moving the legs to get comfortable.