I have lousy oil consumption but that is besides the point. One reason for your sudden increase in oil consumption is this. Over the course of normal driving, especially short trips (15 miles, engine oil does not get hot enough to totally burn off all of the contaminants (acids, condensation, carbon, etc) that are a byproduct of internal combustion. This contaminants build up in the oil and can actually contribute to an increase in the oil level. So even if your car is using oil at acceptable levels these contaminants can make it look like your car is using less oil than it really is. When you go on long trips the oil is given time to get really hot and stay really hot long enough to burn all of these things out of the oil. If you take a long trip towards the end of your oil change cycle you will notice a bigger burn off. If you put fresh oil in just before your long trip your oil consumption will be much less (assuming no leaks, or abnormal consumption). In addition to the contaminants burning off, the oil itself can burn off, this is most noticeable on long trips. Try using an oil with a higher flash point for long trips.
If you are curious, change your oil, then take you normal trips (15 miles each way until you reach about 3000 miles, then take a 700-1000 mile trip and measure your consumption. Then change your oil and drive back home, measuring oil consumption again. I have done this several times and I always use much less oil on the way home.
1995 E420 - retired
1986 420SEL - retired