Major problem

Hi everybody, Please help me with this. I thought I had read somewhere that when dbts become large enough they lose their scutes. Is this true? If it is not, my dbt has lost one scute, does that mean she may have shell rot? How can I tell? If it’s normal, is it supposed to bleed when they lose their scutes? Please tell me what to do. Her shell has been sensitive for about 4 months now, but I thought she just hit herself on something…now she’s lost a scute and it’s discolored. Thanks, Elaine