I understand your worry.
At 20 weeks, a low lying placenta reaching the internal os is not a good sign. As said by your gynaecologist, you must take extra care of yourself now. Give adequate bed rest
. Do not do strenuous activity.
You need frequent ultrasounds to be done to monitor the level of the placenta. Go for regular follow up with your treating doctor.
I am glad that the rest of the parameters are normal.
Be relaxed and less worried as these can worsen the problem.
All the best.