contractions or something else?

I'm 16 weeks pregnant. It's my first time to be pregnant. I often have this tightening of my stomach and back ache.. usually if I walk or do paperworks. The pain would usually last for a maximum of an hour, it would subside for a bit and then ache again.. I get it daily usually from 4pm till I fall asleep. My OB gave me duphaston and isoxilan to take. Is this normal? Is my baby still ok? I'm really anxious.