Hello,
You have been taking Minastrin 24, presently you are not getting your periods. Your 
pregnancy tests are negative. If you miss tablets or if they interact with any other medicine the efficacy may get reduced you could get pregnant. Always start the pack as instructed by your heath care provider or in the leaflet instruction. The more tablets you miss, the more likely you are to get pregnant.
Get clinically examined by a gynecologist, get USG (
Uterus and fallopian) tubes done to rule out ectopic pregnancy. Many women have spotting or light bleeding at unexpected times, or may feel sick to their stomach however do not stop taking the tablets. The problem will gradually go away.
On the days you take two tablets, to make up for missed tablets, you could also feel a little sick to your stomach. In case of 
vomiting, use a back-up method (such as condoms and spermicides) until you check with your healthcare provider. Lack of 
menstruation can be due to 
anemia, poor general health condition, 
hormonal imbalance, 
thyroid problem. PCOS, etc. Get examined by a gynecologist, get treated if not pregnant.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Nupur K., General & Family Physician