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If this Is the first episode of you having this problem with menstruation, then you need not panic. It happens quite common among young girls. You need not worry about it now. Wait for some more time and I'm sure you get normal flow of menses. You can take a course of pain killer tablet for your back pain like paracetamol 500 mg whenever needed. If your periods are irregular for more than 3-4 months then consult with a gynaecologist. For now, wait for few more days and watch for a normal menses to start.
Hope this helps. if you have any further questions, feel free to ask
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