Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
How long has the lump been present prior to opening?
When it opened, was there pus or blood?
If possible, could you take a photo of this? If you can do so and post them online, I am happy to have a look (since it will let me see what you are seeing). To post them, you can either use the wee paper clip on the tool bar (How To). Or you can post them on a 3rd party site (ie Flickr, Photobucket, etc) and paste the web address here.