"Chris 1971"
Was it a rule doctor or a rule clinic requirement to have a chaperone member of family in the exam room even for teenagers of 18 age or was it only your step mother who wanted to come with you and and watch at the exam ? But she could have waited in the waiting room ?
In that case, why you did not complain, told her that you were too old and it was too much embarrassing being examined in front of her and if she did not agree, told it to your father ?
Did you have to remove completely the gown and stay naked for a while under her eyes ?
Did you have to bend over, and spread your butt for a visual exam of the doctor in a spot which let her see your rear and butthole during the exam? Or were on all fours on an exam table ?
Was it worse with a finger inserted inside your rectum ?
Was the doctor ,(you do not say if it as a woman or a man ?) conscious of your embarrassment and trying to protect a little your privacy by not exposing you too much to her view or on the reverse not minded it at all ?
Did the doctor asked at your stepmother why she was present if it was not required and surprised by her coming ?
Did she looked away sometimes during the exam or on the reverse was permanently looking at your exam directly and ask questions to the doctor ?
Did she commented your exam after while coming back home ?
Did she related it to your father or not ?
In presence of other siblings or not?