I had to get a physical to play sports when I was 14... The doctor pulled my shorts and underwear down and felt my scrotum and penis, then did a hernia check. It was all fairly quick but in full view of my mom. It had been a while since she had seen me naked and I think she was quite surprised to see that I was well advanced into puberty and had quite a bit of pubic hair and a mature penis. I think she had still thought of me as her little boy!
That's precisely the situation to avoid. If your mother had been witnessing your genital exams every year like clockwork, there would have been less or no surprise. If she went from seeing you at Tanner Stage I and then not again until Tanner Stage IV, it's no wonder she was shocked.
I was a late bloomer, so for most boys, this calendar happens two years earlier, but my pediatrician showed my mom at my fourteen year old check up with the orchidometer beads that my testicles had started to grow (Tanner Stage II). Then at my fifteen year old check up, he showed her that my pubic hair had finally started, and reassured her that my penis was growing too, even though I had gained weight, and the fat pad made it seem like my penis still hadn't started growing (Tanner Stage III). Finally when he lowered my undies at my exam at sixteen, she saw my full adult sized penis (I had slimmed back down) with significant pubic hair (Tanner Stage IV), which was the most change she witnessed in any single year, but given the trajectory of the prior two years, was not shocking.
We changed doctors, but had she been at my exams in my later teens and seen me at Tanner Stage V (adult pubic hair pattern), it would not have been a big deal for me, since she had seen me all the way through puberty. Again, if you subtract two years for most boys, with the above path from Tanner Stage II to V happening between twelve and sixteen, it's all very reasonable and why so many places have that policy about having a parent present until sixteen.