usually in early pregnancy
placenta can be low lying and in will migrate upwards with progression of gestation but in your case as it is completely covering the os the possibility of complete migration is less,
so you have to be cautious as told by your doctor as sometimes the bleeding can be heavy that can affect the health of both mother and baby.
pills usually will not affect already established pregnancy,
at present you have to avoid travelling and intercourse as told by your doctor and also take rest as much as possible, avoid lifting weights, let your family members know about your condition and if bleeding starts immediately go to hospital.