losing oil on 2001 Cadillac DeVille DTS

after driving approx. 100 miles on a new oil change the check oil light came on. was in the boonies & drove about 3 miles. checked oil and was only about a quarter inch on the dip stick. what could be my problem

check for leaks at the filter and drain plug.did they put enough oil in?
oil in water but not water in oil