Thanks for writing the query.
As you provided the detail,it seems that root canal procedure was not done properly,therefore there is re-infection in the same tooth.
There is abscess
formation in the periodontal tissues in the same area,if left untreated may lead to loss of underlying bone.
There are to options of treatment in this case,first is re-root canal procedure by an expert and the second is apicectomy of the same tooth by an expert.
So i would suggest you consult an endodontist at the earliest and get the re-root canal treatment
done,during this procedure you have to have lots of patient because as you said you are already immuno-compromised healing procedure may take time.
Nothing to get worried.
Hope this will be helpful.
Dr. Gunjan Gupta