How to deal? Inappropriate touching...

My 5 yr old son i just found out today, was caught doing a few inappropriate things by his baby sitter.

Her 4 yr old son was leaning over the railing on their porch trying to reach the toy that had fallen, and his privates slipped out of his underwear slightly, and my son decided to take this opportunity to to grab his privates, and my babysitter thought she heard him say "tickle, tickle".  She blew it off thinking maybe he meant to grab his thigh, and went to far.    

Then later on, she ran into the house to use the restroom, and came back out.  She called out to the boys who were behind the garage.  She went behind her garage,  and saw my son with his pants pulled half down, showing off his private parts.  The younger child was also in the process of pulling out his privates when my sitter found them.      My son freaked out, and she finally calmed him down and spoke to him about it.    

I don't know how else to go about handling this situation.   As a recap, when my son was 2, he was inappropriately touched by a 5yr old boy of another sitter he was going too.   Could he remember what happened to him and be replaying it with other boys??     I had an issue with him at school this past fall where he decided to show off his privates IN the CLASSROOM with a small group of boys.    

All of this is just making me really worried that its all stemming back to when he was two.  Should i seek additional counseling for him??  Or is this just normal behavior for a 5 yr old????