Hi.. Thanks for the query.. A huge sore on the roof of the mouth in the area where you were given anaesthesia injection can be due to soft tissue injury and infection.. Many a times there can be hematoma formation due to piercing of a blood vessel with the injection needle leading to a painful lump.. It generally heals on its own and you need not to worry about it but in case if it is painful you should consult your Dentist and get examined.. You can be advised to apply a numbing gel over it especially before meals so that it eases the pain and eating becomes easy.. Also Ibuprofen can be advised to reduced inflammation and pain.. As you are already on antibiotics, you can continue with the same but also start applying antiseptic ointment like Chlorohexidine for fast reducing of infection.. D warm saline gargles. Hope this helps.. Regards.