Unexpected doctor's visit

This all started the weekend the boys turned 6 months old, in October. They got their shots on Friday at their check up and we went to Grandma and Grandpa's afterwards for the weekend. The next morning when I changed Ayden he had a spot on the back of his left leg. The spot was purplish, hard and sort of linear shaped going across the back of his thigh. It was more of a whelp than a sore but you could see a tiny little spot in the center. We showed it to the doctor a couple weeks after it appeared and he said to keep an eye on it and come in if it changed or caused him problems. The spot has not really changed much in the past two and a half months. A few days ago it started to change, it got a circle in the center. Yesterday afternoon when I put Ayden on the changing table I noticed the spot was oozing thick yellowish-orange puss! I tried wiping it away and more came out. I tried to squeeze it out but that made Ayden REALLY mad! After the puss came a little blood and then you could see a tiny little hole in the middle of this spot. As I continued to change him the puss lightened up and turned a little more clear. I put a band aid on it to keep the puss off his clothes and put the boys in their car seats. We were already going in to town to visit my old school. I called the doctors office once we got in the car and I got an appointment for a couple hours later. After visiting with my old co workers and some former students and their families, we headed to the doctors' office. The doctor tried to squeeze some puss out also, again, Ayden got REALLY mad. I don't think he really knew exactly what to say. He told me he thought it had been a spider bite (what I thought from the beginning), and that for some reason it had a hard time healing. For some reason recently it had got infected and was now pussing. He gave us some antibiotic ointment to put on the spot twice a day for five days to clear up the infection. We already had an appointment in about two weeks for their nine month check up so he will look at it again at that time. The spot is already looking better, it is still purplish but it is not swollen or hard anymore. I hope it continues to clear up and doesn't cause him any more problems! On a side note, my little man continues to put on the pounds! They weighed him, with his sweater and shoes on, and he was exactly 19 lbs! What can I say, the boy loves his boobie and his food!!