If you have a recent save you could re-load and make sure the level-up jug is not the last one you check.

I don't know if this would help, but I'm pretty sure I left the 6 gold keys in the inventory files for the imp's house key;
Lost Items - guide and directory of inventory files

Other than re-loading, iZakaroN's SaveEditor (or from Kiya's site, with description) would be an easier way to get the level up you missed, though. The experience required for each level is;

E = 1000*(level^3 - level)/3