ds has pain in groin area

for the past week my 20 month old has been guarding his groin when i change his diaper. at first i thought it was a control thing, cuz he doesnt like getting his diaper changed, so i thought he had just found a new way to block me. he presses his legs tightly together so i cant wipe, most often after a BM because he also has been having some normal toddler privacy issues about BM where he doesnt want me to look while he poops, etc.

anyway it doesnt seem to be his penis, but more like his testicles , or the skin of his scrotum. he has had diarrea for over a week too...we had the stomach flu last sunday and since then he really hasnt had a normal BM.

the skin doesnt really look red, although today it was a little. he has ben slightly rashy, and i am now trying to watch what he is eating to avoid acidic stuff.

he was really acting like it hurt today, whereas before today he was just guarding. but today, it looked slightly rashy so i dont know if the week long guarding was due to pain or not.

is this a normal toddler thing, to not let mommy wipe? i am concerned but not overly, and not sure if i should be more worried and call a doc.