Odd red bump on stomach

Can anybody give me an idea as to what is happening here? This is on my girls stomach, I noticed it a few days ago and it has gotten significantly worse. Where the dark spot is on her skin, there is a actual hole forming. It doesn’t seem to hurt her and I haven’t noticed any changes in her eating/drinking It’s hard to tell but it is a couple inches above her spay scar, right above her tattoo. I thought it was maybe a stuck stitch trying to make its way out, but that doesn’t seem to be the case anymore.  I called our vet and they can’t get us in until next Wednesday, so any advice or knowledge would be appreciated. I’m very worried this is something serious. 

Hello!  That was also my first thought, about some dissolvable suture material (often the knot) working its way to the surface.  This would not be uncommon. What makes you think that this no longer appears to be the case?  Other possibilities include cancer (which is often non-painful) or an abscess or wound (which is often painful), but don't make diagnoses based on perceived pain-levels as people frequently get this wrong, and not every pet has read the text-book before-hand anyway.  If your vet has triaged the case as relatively low-concern, this can only be a good thing, but we would advise keeping on eye on it and not hesitating to contact them if it gets any worse.  Constant licking can prove to be a real problem in such cases, so if you see that the lesion is being licked, let your vets know in case a cone is needed while you wait.   Please do let us know how you get on.