From my experience, if a hen lays normal sized eggs then all of a sudden lays an undersized egg it is usually yolkless if you were to break it open. I have never had any of my tiels to lay an undersized egg, but I have known of some of our chickens to lay eggs like this from time to time. Normally as the ova mature in the ovary, a yolk will form around them. The ovum and yolk are then released into the infundibulum of the oviduct to be fertilized by the sperm the hen has stored there (if she has mated). What happens in a yolkless egg is that the ovum/yolk fails to be released from the ovary into the oviduct and it just doesn't make it into the egg. A piece of tissue is sloughed off from the ovary instead and that stimulates the formation of the albumin which is then encapsulated by a membrane and then the shell before passing from the oviduct and out the cloaca (vent.) These eggs cannot hatch because the ovum is attached to the yolk in the ovary. Without the ovum/yolk no embryo will develop.
*ova: female sex cells; plural; more than one.
*ovum: female sex cell; singular.
Oh and to answer your question on whether she is okay or not, I think she should be fine. I'm sure there are reproductive diseases/disorders that cause hens to lay small yolkless eggs, but unless she continues to lay the undersized eggs I don't think you have anything to worry about. We have hens that lay at least one tiny egg every breeding season and we've been breeding them since they were pullets. If you feel there may be something potentially wrong, have her checked out by a vet just to be certain.