I am no expert, but being married for 23+ years last October 9th, it gives me a little insight.
Question is she wearing a new pair of shoes or boots?
If no have you checked the closet or the trunk of the car?
If the new shoes arent there then it means that she is squirrelling away more money for a really expensive pair of shoes, be prepared for more mystery math lessons.
If it has nothing to do with shoes its probably gas.
Odds are its shoe related.
PS
Shoes are the drug of choice for women, the more uncomfortable the better.