That one flower may have been the one to bloom last, idk. Unless you have seen this plant's buds bloom, and remember the order in which they bloomed, you will never know for sure. And as stated, unless you have had the plant prior to the buds going into full bloom, you will never know how long those flowers have really been in bloom for.
Not to mention, it's also quite possible that different blossoms have different lifespans, they may not all wither and fall off within the same exact span of time. For example, blossom #1 may last 10 days, blossom #2 may last 12 days, but bloom #3 may last 11 days; but in general the flowers on the plant would last between 10 - 15 days.
Because you really don't know with 100% certainty whether those blooms are fresh or not, or how long they've really been in bloom for, you will not be able to get an answer that is 100% definitive from anybody. We can only speak in terms of possibilities and probabilities on this matter.
The consensus so far has been that this is most likely not
the result of the repotting, and that it is highly probable that this is a natural withering of the blooms, unless further evidence is provided somewhere down the line to prove otherwise.
On the bright side, you still have one flower to look at before it goes out.