Is he neutered? I take it he is, but if it has been a recent neuter, then male cats can have what is called "stud tail" which is caused by hormones, and that area can be tender and usually it looks like the tail is matted with fur and oily or greasy looking. He may just be biting at his paw, because he is referring his pain and frustration to that area.
But, he could have fallen, or gotten his tail stuck in a drawer or door, and maybe it has a bruise that is hurting him, that is causing the swelling, and perhaps, if he did fall or get stuck in something, his paw may be injured too. I would get him checked as soon as you can, if he doesn't seem better by tomorrow. If he starts growling or won't let you touch him, I would take him to an emergency vet, especially if you have other animals, that may have gotten into a fight with him. If he stops wanting to eat, or drink, or use the litter box, definitely keep an eye on him, and call your emergency vet, or your regular vet in the morning.
Just make sure he is not bleeding or getting sick in any other way, like vomiting or having diarrhea.
If the area that is swollen is under his tail area, near his rectum, he could have infected anal glands, that may need to be "expressed". They can get infected and cause pain.
God bless and take care!
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Elena Marie
"Open your heart to learn all you can in this life, and to give to others, the joy and satisfaction that they may remember you as a true friend."
