I agree, that this is most likely a milestone thing. 13 months is a prime time to start walking, and there are always sleep issuesduring those times. It sounds like you are okay with letting her cry it out, and if you can stand to do that for 3-4 days, she should go back to normal (that is what my son has done during his milestones).
You also mentioned that she wakes up crying in the morning and gets a total of about 2 hours of naps during the day? You may want to look at that too. I have been told that you know your child got enough sleep if they wake up happy. My son usually wakes up talking and babbling to himself, and after awhile, may get upset that I am trying to sleep a little longer, but it is usually a happy wake up. You may want to try putting her to bed earlier, depending on when she wakes up. Also, I have read that naps should be at least 1.5 hours and no more than 3 hours so that they can get the full cycle of sleep in. You may want to try combining her naps? Are you waking her up, or does she only sleep for an hour? Is she waking up crying from those? You can also move those earlier if necessary. My son is 16 months, and he usually gets about 13.5 - 14.5 hours of sleep a day. I don't know if this is truly an issue or not, so sorry for offering unsolicited advice, but my ears always perk up when I hear that kids wake up crying.