I agree that it does not sound like an abscess. From experience with my guinea and from what I've read, abscesses grow very rapidly. Mine appeared over night and grew very large within days. I read that an abscess will grow that quickly. It does sound like a fatty lump. Fatty lumps are similar to an abscess. I read so much on guinea lumps when mine developed his, and except for the rapid growth, a fatty lump was much the same. I by far not an expert on lumps, but I read that a tumor or cyst would cause the guinea to look or act sick. (like not eating or drinking). That's how I knew my guinea didn't have a tumor or cyst. He was to perky. But your guinea needs to be checked by an experienced vet, who can immediatley tell you what is wrong. Have you checked in the phone book for a doc that specializes in exotics? That's how I found my vet. She was one of the only ones in the area, but she knew her stuff. The minute she saw my piggy she said, looks like an abscess (which it was). If it turns out to be a fatty lump, they may not have to remove it, unless it's in a spot that hinders the guineas everyday activities. It seems like your guinea has had the lump for a while with no problems. So I take that as a good sign. Guineas go downhill so quickly when they are sick. I hope everything is ok with your guinea. I get a little crazy with reading articles, but when my piggy got his lump, I searched all over the internet for any info I could get on lumps on guinea pigs. And got so much information. That's how I stumpled on this forum and I'm so happy I found it because it has been a blessing for me! Such supportive people have helped during the time that my guinea pig was sick. He is still sick, but healing nicely. I have recieved wonderful information on how to take care of my sick guinea. I do hope you get the help for your guinea. If you type under search on the internet "guinea pigs and lumps" you can't believe all the info you'll get. It took me to so many websites. I got so much information on lumps and what it could be that I figured out it was an abscess before I went to the doc. Like I said, since your little piggy has shown no syptoms of illness since the lump appeared, I'd say that's a very good sign, but only an experienced vet would know. What is your next move with your guinea? I wish you the best luck. Please let us know how he progresses.