I work as a CFHN and immunise. We always tell parents of the risks of intussusception although it is very uncommon from the Rotavirus vaccine (Rotarix where I am from) but it still increases the risk hence why to warn of signs of it. The signs we say to look out for are extreme pain/distress, bloating of the stomach/stomach distention, constipation or straining or vomiting unusual stomach contents or extreme frequent vomits. More frequent bowel movements is not a sign of intussusception.

I did do some work in a different facility and noticed at the time (immunisation clinic) that not one nurse in the 5 others I worked with warned of risks of intussusception. I was surprised.

From what you have written it sounds extremely unlikely to be intussusception but your DS change in behaviour is definitely likely to be as a result of his immunisations.