I wouldn't expect to notice a change in fuel consumption between oil changes, much less the 60% increase described.
First, 50 km is not a very long trip; the difference between 9 k/l and 5.6 k/l is only like 3 l. Are you sure your fuel consumption measurement is accurate?
Next, was there anything different about this recent trip that may have caused an increase in fuel consumption? For example, hilly terrain, A/C on, lots of idling, extra weight in the vehicle, carrying something on the roof, parking brake engaged, towing a trailer...
If not, check for leaks: inspect fuel lines (with fuel pump running), fuel tank after filling up, etc. Often you can detect a fuel odor after driving.
Could be any number of things, or a combination. Things like extra rolling resistance: Low tire pressure or a dragging brake. Or less common things like a slipping clutch or automatic transmission.
Time for a tune-up, maybe?