Your first few AF's post baby will always be out of whack and different. Sometimes they vary in length and strength and sometimes in the amount of time in between. Its unfortunate, but it does happen. When DD2 was 18 months old I got my first AF, and then I had them fortnightly for a few months!! It was horrible.
I never really used to get pain, just a mild ache for an hour or so, but my first period back after having a bub was more painful than usual. And my cycle still isn't regular, although I've only had...2 or 3 since bub. So, as Christy says, it can take a while for your body to find its rhythm.
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