What Causes Itchy, Bean-sized Subcutaneous Lump In Right Posterior Shoulder?

Two days ago, I noticed a very itchy, bean-sized subcutaneous lump in my right posterior shoulder. Didn t think much of it, initially, since I have a very damaged, arthritic right shoulder - with lots of crepitis and fragments - from a major injury sustained in a 1968 Vanderbilt football game against Ole Miss. However, after 3 days, the lump increased in size to the size of a peach pit. I also, at that time, began to notice a small red, pimple size swelling on the surface of my skin right over the peach pit. I became more concerned and researched itchy hard lumps shoulder back. Saw a whole bunch of possibilities, which prompted me to seek professional medical care - either an orthopedist, a dermatologist, an oncologist, or all three! Rather than start a lengthy phone shopping campaign just to get an appointment with just one of those physicians, probably in a month or two at the earliest, I decided to take myself to an ER, where any of those specialists could be quickly summoned. After x-rays and a visual and manual examination, the ER Doc said she was fairly certain that it was simply a spider bite. She prescribed a 10 day course of oral antibiotics which I started last night. It is now a day later. The site feels much better. But, the underlying hard mass is much, much larger. It has at least quadrupled overnight. The ER Doc said give the antibiotics at least 3 days before I should begin to get concerned. So, that would bring us to Monday. Oh, by the way, I am a 68 year old male. Should I wait til Monday if things get worse instead of better? Or, should I go to another ER over the weekend, since that is when doctors offices are all closed? Don t want to panic here, but for an old fart like me, early detection and treatment are key to resolving most medical problems.