Back arching, crying and kicking out legs can be signs of reflux. My baby boy did the same thing starting at about 3 weeks old and it turned out to be reflux. There are lots of different things on here about reflux. Have a read and see if anything else about reflux sounds like your bub. I may be wrong, just a suggestion. My boy was happy and sleeping perfectly up until that began and it took ages to convince me it was reflux! After 6 weeks of medication, he was back to normal and happy again. He's now 6 months old (today!) and is still on Zantac but hopefully not for much longer. He used to make grunting sounds as well when he was in pain. Good Luck!