Choke tube swelling happens from time to time... Nothing you can do other than to get a new choke.  My advise is to ditch the undertaker and get a JellyHead if you are on a choke budget.  If you can spend more and the JellyHead doesn't work for you, get a choke from either Pure Gold, Indian Creek, Kicks, Sumtoy, etc...  I personally am partial to Pure Gold because of performance, strength and outstanding customer service.