Trigger to run only after checkbox goes rom unchecked to checked - triggers

Question

I want to run a trigger on Contact if a checkbox on Contact object goes from unchecked state to checked state and then do some processing on the records. I have this trigger on after insert and after update. The code works fine for update but if I check the box while creating a new contact record, the trigger does not detect the below condition in the code and therefore sends an empty list to contactTriggerHandler class.
if (Trigger.oldMap.get(con.Id).checkbox==false && con.checkbox== true)
trigger Contact on Contact (after update, after insert) {
List<Contact> listofContacts = new List<Contact>();
List<Contact> usersToCreate= new List<Contact>();
if (Trigger.isAfter) {
if(trigger.isInsert){
System.debug('After insert');
if(RecursiveTriggerHandler.isFirstTime){
RecursiveTriggerHandler.isFirstTime = false;
for (Contact con : Trigger.new) {
Contact contacttoUpdate = con.clone(true,true);
contacttoupdate.Nickname__c=contacttoUpdate.firstname;
listofContacts.add(contacttoUpdate);
}
System.debug(listofContacts);
ContactTriggerHandler.AfterInsert(listofContacts);
update listofContacts;
}
}
else if(trigger.isUpdate){
System.debug('After insert and update function');
if(RecursiveTriggerHandler.isFirstTime){
RecursiveTriggerHandler.isFirstTime = false;
for (Contact con : Trigger.new) {
Contact contacttoUpdate = con.clone(true,true);
contacttoupdate.Nickname__c=contacttoUpdate.firstname;
listofContacts.add(contacttoUpdate);
if (Trigger.oldMap.get(con.Id).checkbox==false && con.checkbox== true) {
usersToCreate.add(contacttoUpdate);
}
}
ContactTriggerHandler.AfterInsert(usersToCreate);
update listofContacts;
}
}
}
}
Here is the recursive handler class:
public class RecursiveTriggerHandler{
public static Boolean isFirstTime = true;
}