Sometimes i blank out and apparently do normal things, but i can't remember any of it. why does this happen?

Normal vs. abnormal?: Not uncommon for any individual have moments where focus is altered such as i often hear from patients who will say they've driven a familiar rout with lights or stop signs and can't recall if they stopped or not :o regardless, the frequency of these events and the stress weighing on their minds often account for these. Work up for add or petit mal sz. Or other cause is a clinical decision for you.

Possibilities: There are many reasons this could be happening. Among them are seizures and dissociative episodes; simply being not paying attention to what you do is another. Please see your physician about these episodes, so that you can receive an evaluation & treatment as needed. Neurological and psychiatry consults may be part of the workup as well. There is help, though.