Going 6 weeks

I tested positive on a pregnancy test kit .. I tested 4 times at different days of the week, after I missed my period. Now I am going 6 weeks from my first day of last menses. I dont have common pregnancy symptoms like morning sickness and all. I only just experience metal taste in my mouth at times.. But recently I experience some aching pain on my lower back.. mainly on the left side.. is it serious? I have never really felt cramps during my period previously. Now I do sometimes experience cramp pain. I have not experience spotting or bleeding... Is it anything serious? When should I see the ob for ultrasound. Anyone has the same symptoms or experience similar situations?