I believe vitamin D deficiency is linked with hair loss but you would have to research recommended suppliments. Your pharmacy can probably advise you if it is not serious enough to consult a doctor.
are you breastfeeding? if so, you choice of supplements could be influenced by that. Its common, for up to 6 months after childbirth, to have thinning hair, especially in such a hot summer where we all seem to be moulting like cats round here. Its probably your hormones settling down but maybe you could have your thyroid levels checked too, just in case they are out of whack right now?
I’m 12 months post partum I lost loads of hair. I got hairburst vitamins for new mums and it really brought my hair on. Now I have loads of hair growing back on my hairline which I’m delighted about but can be hard to style!! I got them in Holland and Barrett