It could be taken negatively or positively depending on how your day went but it's the feeling after you had a really bad day or a really good day and you just lay down and melt away but at the same time become whole again. You disconnect with the world and connect with yourself.
I'm feeling sand/Sandy because I need to get more connected with myself. It's the feeling when you lay down and disconnect yourself from the work which makes you commect more to yourself.
This was originally slang used by soldiers in the British Army. It probably dates from the 1920s and was in widespread use during World War II. A sand rat is a whore, hooker or prostitute in a seaside town. These women were not based in a bordello but walked a pavement ‘beat’ (see also pavement fairy) and were known as sand rats because, like rats, they only came out at night and would take their clients down to the beach for sex on the sand.
A British squaddie going on leave would be asked by his mates what he intended doing. His itinerary would almost certainly include the phrase ‘sand ratting’. This meant that he intended to engage the services of a sand rat.