I have been told that you should never check you tyres once you have driven your car, only when they are cold. The resaon being that the further you drive and the hotter the day the more pressure you will have in your tyres.ie hot air expanding.
1 check all pressures when cold with $20 gauge from kmart and remember how much each are down.
2 Drive to petrol station if you dont have a compressor at home, add the correct amounts to each tyre and check with your own tyre gauge.
This way you will know for sure, how much of a leak you, if any!!
ps down 20psi i think you have a nail
Good luck