Is this m?

你幾多歲?
假如4日都有的話..可能係m喇
同埋第1次黎多數係咁..
唔係個個都會有經痛..
有經痛同累都係個人既..你冇依d唔代為唔係黎緊m
可以參考下以下網址
http://www.hkma.com.hk/chinese/pubmededu/special/menstr.htm

Probably yes. "M" is blood from menstruation. When blood is dried, the colour will turn from red to brown.

For personal hygiene , you are strongly suggested to use sanitary towel.

I think it is.