Welcome to healthcare magic.
I agree with medical store guy. I don't see any reason for you to panic. It could be a bruise
. It will resolve in a matter of days. Other possibility is contact dermatitis
to the glue of the plaster but then you should feel either itching or burning sensation. The antiseptic on the band aid can stain the skin as well. So I will recommend you to just wait and watch.
Hope this helped,