Thanks for the query.
According to your history the possibility of pregnancy
is very less.
Menstrual periods can get altered due to many factors.
Stress and strain, dietary changes, change in physical exercise, anemia
, thyroid hormonal
abnormalities etc. are some of the causes that can affect the menstrual cycle.
So you once consult gynecologist, get examined to rule out above factors.
And with her advice you can go for tablets to induce withdrawal bleeding.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429