Thank you for asking your question and welcome to Healthcaremagic.
I can understand your concern, it is very good of you that you asked us this question. Many people are shy and hesitate to ask such questions which will ultimately lead to some or the other problem. Your daughter is 13 years old as you have mentioned that means she must have attained menarche(Started having periods) quite recently, I guess around one year ago. During the initial years, I mean for the first one or two years girls do experience such problems of irregular and frequent periods, that itself is not a major problem to be worried about.
The issue which needs to be addressed is her weakness, I would advice you to get her examined by a gynecologist as well as a physician, because frequent periods mean more bleeding, which causes anemia
. which could be a reason for her feeling weak.
Please do consult your doctor.