18 weeks pregnant and light bleeding

Sara
Throughout my pregnancy so far I have not experienced any bleeding 
I woke up on Friday morning and I was bleeding light pink. No painor cramps
Had an ultrasound done and baby boy is fine and placenta is nice and high  but they don't know what caused the bleeding. Doctors don't seem concerned. It still hasn't fully stopped, I was wondering if any of you ladies had experienced this ? 
Could it be from me doing to much in work ? I work in a hospital working with elderly mental health patients and they can be very heavy and hardly do anything for themselves 
We are first time parents so after some advice
Thanks!