Pulling legs up

So for the last 2 days my boy has been bringing his legs up to his chest and grunting really hard. His face turns red and it only lasts a couple seconds. He doesn't scream in pain but the grunting makes me think he's uncomfortable. The grunting and bringing up of legs never produces any gas so even though I think he's pushing like he's trying to pass gas, it never happens. He isn't constipated he's formula fed and has pooped 2 times today and 2 times yesterday, which is normal for him. I don't know why he's doing this but I'm worrying because he brings his legs up about 3+ times a minute. I keep reading about intussusception, he has the rotavirus vaccine about 3 weeks about now but I'm still starting to freak about it. Any insight would be great.