Is the Apostle Paul saying here that nuns have to always be nuns or they bring judgment on themselves?

No - This is referring to young widows (women whose husbands have died). Look again at verse 14: "I counsel younger widows to marry." Paul does not want young women to be added to the list of widows who need to be cared for by others. He is advising them to marry and carry on with life, avoiding all of those bad habits that he lists in verse 13. It has nothing to do with nuns.