Thanks for sharing your concern.
Do not worry if your dentist
has examined thoroughly and suspects pus in your oral cavity then she must have drained them too.Any pus anywhere in oral cavity should be drained well before further treatment.
Also you have already started with your antibiotic dosage due to heart murmur
,so even if there is any infection and pus remaining, your sufficient dosage of antibiotic will help in this case.
just take your medication properly in this case.
Now the blister
that you would have seen could be pitting edema(swelling after extraction which generally occurs).
For pain relief you can consult your physician and take suitable analgesics for three to five days.
I hope the symptoms should subside within five days, if not then you need to visit your dentist for follow up.
meanwhile take all precautions and due care for post extraction healing.
Take good nutritious and semisolid diet, and relax you will be fine soon.