Hello, well from the history you have given, some women do experience weakness once menustration starts....These are due to hormonal
changes occurring in the body....Anaemia
usually occurs if you have mennorhagia (bleeding for more than the usual days or profuse bleeding).....And you said you recover by 3rd day
..Usually in anaemia the weakness still continous..But it is always better to have a check up by your gynaecologist and to do a haemoglobin
estimation and if required a thyroid
profile just to rule out in case there is any underlying payhology. ...hope i was helpful to you. ...