Vaginal infections with female Beagles can be due to bacteria, yeasts or viruses. Aside from a bad odor, there can be itching and/or some discharge (ranging from light to heavy). With some Beagles, the smell can spread to any place that the dog sits or lies down on like the sofa, bed, carpeting, etc. Even if the discharge is not noticeable, the odor can linger on fabric for quite a while.
This can be successfully treated with a course of prescribed antibiotics.
In any case, a fishy type smell coming from the back end or vagina of a female Beagle is not normal. It does point to a medical (though not typically very serious) condition.
We should mention - for the sake of other owners - that a bad, fishy smell from the dog's vagina could be pyometra. Though many people think that pyometra cannot happen to spayed dogs, there are cases of it developing in the uterine stump. Though, in your case since this was happening before having her spayed and continues to happen now, this should not be a concern for your Beagle.