There is a lot of bleeding under a crown when i brush at the gumline, what should i do?

Get it checked: Bleeding at the gum line may be a result of plaque accumulation due to poor oral hygiene, the presence of calculus, periodontal disease, a carious lesion, an open contact between the teeth causing food impact or an ill fitting crown etc.

See your dentist: Bleeding is a sign that there's something wrong in the area of that crown. It could be something simple like gingivitis or a local inflammation of the gum requiring only a cleaning or it could be something more serious like a large cavity that could potentially lead to the loss of the tooth if unattended.