The bandaid is considered a chatzitzah, a barrier to immersion. Since it is small, and it is something that was hard to notice, it was a rabbinic-level barrier.
Standard practice in this type of case is to immerse again, and this is what we recommend in general.
If, however, there is currently high risk of infection from the Coronavirus pandemic in your area, please get back to us or a local halachic authority to discuss best practice.
Please let us know if we can be of further assistance.