I found a red lump on my dog. Should I be worried about it?

Ouch! I'd have that checked out by your vet in person, especially if it appeared suddenly or is bothersome to Bella. They can take a sample of the spot to rule out any more serious issues such as tumor growths, infections that need treatment, cysts, or see if it's something benign such as a skin tag or other small growth.

Until you can get into your vet, it's a good idea to keep the area clean and dry, and monitor it for any signs of changes such as oozing debris, having irregular margins, worsening, etc, as these can all help your vet pinpoint the cause. An Elizabethan (cone) collar can also keep Bella from licking or chewing at it which can reduce irritation and prevent a secondary bacterial infection from taking hold.