Demonia Venom

i am very good at keeping track of my linden but for some reason when i have a balance of 0 i can log out and come back in and it will be a - 3 or 5 and on rare occasion 1 this is not right and very unfair please help me find a way to stop this im getting frustradted and about to leave sl because of it. thank you for any information on this subject.