Fussy newborn?

My 3 1/2 week old just started to have very fussy periods every evening/ She sleeps greatduring the day, but after 5 or 6pm she becomes inconsolable- very fussy, doesnt sleep for hours, and eats ravenously (up to 6-8 oz in a feeding). I thought it was gas & tried Mylicon drops with no luck. Is it gas? Colic? HELP!!!