The disease causing the swelling and strange shape and texture of your yellow plum might is probably a fungal disease called Plum Pocket disease,   The key, according to Dr. Jay Pscheidt at OSU is the enlarged fruit, which never develops a seed pod.  Watch to see if the fruit continues to enlarge.  You can also cut open the fruit to see if a seed pod is developing.  The fruit will eventually become covered with velvety gray spores.  The fruit should be removed and disposed of in the garbage.

 Plum Pocket Disease is caused by the fungus, Taphrina pruni.  It overwinters on old diseased fruit, and affected twigs and limbs of the tree.  All affected twigs, limbs and fruit should be pruned out and removed.  Fruit on the tree and on the ground should be bagged and disposed of in the garbage, not compost.  Control with dormant spraying next winter/spring with a Bordeaux spray, Lime Sulfur Ultra  or Rex Lime Sulfur solution, available at garden centers.  This article has additional information,  Plum Pocket Disease  https://www.gardeningknowhow.com/edible/fruits/plum/treating-plum-pocket-disease.htm.