Dd2 started her periods 4-5 months ago. She's had a few since then, more frequently than we'd been expecting. I've been keeping track and she started one today, on day 17. That's her shortest gap so far - the last one was on day 23, and the one before that on day 20.
I thought the day 20 one was too short already, so day 17 has me a bit concerned. I have no experience of very frequent periods in a "new starter" - my own were bang on 28 days right from the start, and her older sister, at 15, is only recently starting to get one a month, after a few years of 3-4 months between periods.
I've bought her an iron supplement in case the blood loss might make her anaemic. She doesn't get period pain, which is a huge relief as dd1's are crippling. Hormones/moods all over the place but at 12 that's to be expected. I wondered whether periods this frequent are normal in the early months? She doesn't want to go to the GP as she's terrified of needles/blood tests, although obviously I'll take her if/when really needed.