Why do some women have menses cycle change?

For e.g. a women is having her regular menses every mth and suddenly she had a mth of menses and nxt mth the menses does not come as regular as before.

It happens. Stress, diet change, exercise change, pregnancy, miscarriage, birth control pills, all these things can effect our cycles.

It's the joy of being female.

Other than those you have stated is there anything else? I don think all those things happen to my girlfriend.

Menstruation cycle can be affected of your heredity.