Thank you for writing. I cannot tell the size of your fruit but there are five possibilities.

No. The University of Minnesota lists several varieties of plum trees that it considers hardy.  No. The Alderman plum is noted both for its ornamental value and for the magnificent burgundy-red fruit it produces. 

That leaves these.

No. La Crescent produces a high-quality yellow plum that looks somewhat like an apricot. In addition to producing high-quality fruit, the La Crescent plum is a vigorous grower. 

The Pipestone plum produces a large red plum, and is perhaps best suited for drier areas of the state. 

If you live in the southern part of the state, you may want to experiment with a Superior. This variety, although not particularly cold hardy, produces large quantities of dark-red plums. 

The Underwood plum provides gardeners in the northern parts of the state with a hardy and vigorously growing tree that will produce a medium to large red fruit.

Thank you for writing back. These plums are small, most likely because the fruit were not thinned. https://homeguides.sfgate.com/thin-plums-44277.html
There are many YouTube videos on thinning fruit to increase size and yield. The same is true of apples etc. 

I believe this is a small Alderman plum. 
Take another look next year after hopefully a less arduous growing year.