Hello Kari - I think you provided your own insight - "chronic IDA due to a genetic disorder called HHT." Remember that when vaccines (any vaccines) are tested, people with underlying conditions are generally excluded from the test population. It is only after release that a pattern of exacerbation may appear, then the research begins. Thus, it is entirely possible that further study will determine one or more of the Covid vaccines cause this reaction in certain individuals. Once known by the manufacturers and the CDC, it can be further researched, and it may be determined that one vaccine is preferred over others for those with HHT, or it may be that you just need to be aware of the possibility and follow up with your provider.

Please make sure your experience is reported in VAERS (https://vaers.hhs.gov/reportevent.html.) Also, please be sure to completely fill out all of the items, not just vaccine ID and reported side effect. Include your preexisting condition, any medications, and your contact info. When this is done, if there are multiple reports in VAERS for the combination of side effect/underlying condition, it gives the manufacturers and researchers a starting point for figuring it out.

By the way, there are VAERS reports of low hemoglobin and/or anemia after other vaccines as well, though I do not know whether other conditions were involved. We are all just currently focused on Covid vaccine.
I hope your experience can help solve the puzzle.
Sue