I don't know that I am one who can speak to this issue with any authority, but the way it sounds, this could land on either side of the "trust" fence.

Four days to a week is a fairly short period for a relationship that is (close to/ more than?) a year old. You got engaged just in December. If this is an isolated series of events then let it go, but if the behavior persists for more than a few weeks there may be cause for concern.

A little insecurity can breed an lot of mistrust if it isn't tempered by common sense. Sometimes a ride is just a ride - even on the back of a strangers motorcycle. Don't jump to conclusions unless you see a definite pattern.