Hello and welcome to the forum. Thank you for your question although we know you are nervous. Please know that they call back to take a closer look to protect you and it doesn't mean something serious. Were you assigned a BiRad score? The term asymmetry simply means that the density is different in each breast. Most often, this turns out to be a normal finding with no serious root cause. However, it has to be checked out. Newly developing asymmetry is closely evaluated. This puts one at an increased chance of developing breast cancer so they take it seriously and evaluate. But remember too that hormonal changes aone can cause this. Times this could happen are near a menstrual or near menopause. So, please schedule the follow up and let us know the outcome and ask any questions along the way. https://www.medicalnewstoday.com/articles/321823#breast-asymmetry-and-mammogram-results
And your covid vaccine would not impact your mammogram.
Best of luck and please stay in touch.