I have to be honest - this sounds really odd and if I were you I'd get to a doctor ASAP. I haven't heard of anything like this happening, and any likely explanations that I can think of make no sense given that you were sitting quietly at your desk when it happened. The only possible explanation I can come up with (and it's a total guess, completely without any medical foundation or precedent) is some kind of internal infection or abscess bursting, but even that doesn't make much sense if you haven't been running a fever, had any discomfort or other signs of infection. The only reason that I'm suggesting it is that when my incision got infected, it did have what looked like a little blood blister on it briefly. (The infection turned out to be MRSA, but fortunately they were able to treat it with sulfa drugs and it wasn't a major setback to my recovery.)
Good luck - I hope you're able to get to a doctor and get it checked out.
