Do you have a shut off valve right after your regulator and before everything else? Close it there. Reset your pressure and see how it holds overnight. If you lose it there, then you have a leak somewhere in the regulator itself, or at the regulator to tank point.
Also, what is your gauge reporting? Less than 500? Then you need to go get it refilled very soon, because it's almost empty if it isn't already and you're living off the already carbed beer to force it out through the taps
Or did it go from like ~800 to 500? I forget offhand, but I know when I initially reconnect my tank after a fill it will be higher than the 500 mark and then settle down. This has something to do with the temperature of the co2 in the tank (I keep my tank inside my keezer, so I don't know how it works on the outside).
I've been battling co2 lea problems for the last 2 months. I went and replaced all the O rings on my kegs. We'll see how things are in a few weeks (my last 2 tanks lasted less than 2 weeks).