'Feeling myself' think more of yourself, being confident about yourself. Feeling good about yourself or thinking you look good. e.g. 'you look good today!' "i know right? i'm totally feeling myself!'

It mostly means you are feeling confident. But in rare cases and certain context it can mean to masturbate. You can use it like this when you are playing basketball and hitting all your shots you say: "I'm really feeling myself." Or something along those lines.