The lump under you rabbit’s chin may be a "dewlap". Female rabbits have them. They use them to pluck fur out of them for their nest when they are kindling (giving birth to young). However, they are also prone to abscesses and if there is irritation or the lump is larger than normal, it would be sensible to take your rabbit to the vet. While the abscess may burst eventually, your rabbit will probably need antibiotics, and the lump may be something more serious.