Spotting in 15th wk

I had an episode of spotting 2 days after I traveled for 3 hrs I got an ultrasound immediately and the dr said baby is ok the spotting stopped after a day but i checked again today and found im spotting again its been 3 days since the last time dr says give it 2 wks and we will have an ultrasound again to determine if its harmless im totally freaking out