Should I be worried if my 13 year old daughter hasn't gotten her period yet?

No. The age when girls get their periods can vary a lot, depending on family history, their size and weight, their ethnic group, etc. Usually, there is no cause for concern unless a girl reaches the age of 15 without getting her period, particularly if she is not showing other normal signs of puberty (like breast development).
Not really. There is a large range of ages for a young girl to reach puberty. If you are concerned, take her to her pediatrician and have her examined . It might be time for a good physical if she has not had one done.