Firstly, you need to try to figure out if your retainer is hurting your gums, or it just affects your teeth. The second case can be a bit painful, and it means that teeth are actually moving which is the desired effect. If the retainer is actually hurting your gums, and the pain does not go away you should make an appointment with your orthodontist as soon as possible.
b) Retainer broke
If a part of your retainer broke and it does not affect you wearing it, you can use it all until your next appointment with your orthodontist.
If the retainer is adjustable, it is possible that jaws did not get used to new setting so quickly, which means the child did not wear retainer long enough. In this case you can try to rewind the screw backwards (counter clockwise) yourself, several times if needed. If this still does not help, you should make an appointment with your orthodontist.
If there is a milk tooth about to fall out that prevents retainer to fit properly, wait for a week for the tooth to fall out. If it does not fall out in a week, please make an appointment.
With retainers that do not adjust it is very important that you wear it for certain amount of time, or if not your teeth can move. If you have a reason to think your tooth moved for not wearing your retainer long enough, please make an appointment.