Thank you for waiting. I'm glad to hear you have scheduled a vet visit because that is what your rabbit needs. I couldn't get a clear enough view from the photos, but I can still give you some ideas of what this might be.
If it feels soft and seems to have pus in it, it could be an abscess. A vet can drain it and then prescribe antibiotics. Another possibility for a soft growth is that it is actually a hernia. A vet can repair that.
If it feels hard, it is more likely to be a tumor. That sounds scary, but there are many kinds of benign tumors. Lipomas, for example, are nothing but fat, and are often left alone. Some types of tumors do have to be removed a biopsy done.
There are other less likely possibilities, but abscess, hernia, and tumor are the most common. If you have more questions, let me know in a REPLY. I hope this will turn out to be something minor.
My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service only after you have all the information you need. Thank you!