Here's why:
* Size: Short-tailed weasels are similar in size to a chipmunk.
* Flat tail: They have a distinctive short, flat tail.
* Other characteristics: They have a long, slender body and reddish-brown fur, which can vary in shade depending on the season.
Other possibilities, though less likely, could be:
* Least weasel: Similar in size and shape to a short-tailed weasel, but smaller.
* Young muskrat: A baby muskrat could be small enough to resemble a chipmunk, and they do have flat tails. However, muskrats are typically larger and have more rounded bodies than weasels.
To get a better idea of what you're looking at, try to remember any other details like the animal's coloration, behavior, or where you saw it.