What Causes Vaginal Bleeding During The First Trimester Of Pregnancy?

I am six weeks pregnant. Everything was going well till today. BUt yesterday I was some spotting on my panty. can u please tell me why this happened. I am 35 years of age. My doctor has prescribed me Folomega, , Loprin -75 and Robigest 200 and Thyropace. Are these safe medicines during pregnancy. and why this bleeding happened. IN my first pregnancy it does not happen.