Sounds like bad news, sounds like the head gasket. Check the oil cap for a white mayonnaise type substance and check your coolant for droplets of oil. The loss of oil is concerning, are you also loosing coolant? Lexus's don't use oil, not a drop.
Do a search on IS220d head gasket problems on this forum. There's a design fault with the head gasket and Lexus have been replacing engines under an extended goodwill warranty, 6 years old or under 111'000 miles as long as you have a full service history, but as your car is at 116,000 miles I think you might be outside this.
Good luck