To me, it sounds like it could be an iron related thing. I get a similar thing. I increase my foods with iron when I'm due. I used to take FeFol (iron and folate tablet) but found that it made me constipated last time. Perhaps talk to your GP and ask for some bloods to see what is happening next time.
I agree with MG. Perhaps request a blood test to check your iron levels for anaemia. If you have a history of depression post natally or you suspect a hormonal imbalance maybe ask for a thyroid test at the same time... you might as well. Let us know how it goes.
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