Is it constipation or a growth spurt?

Hello,

My little boy turns 6 weeks old on Friday and for the past 4 weeks we've endured sleepless nights due to his grunting, straining and stretching. He also suffers from reflux so he's on infant Gaviscon with Aptamil First Infant which is helping with his regurgitation. Last night he grunted, strained and stretched continuously from midnight until the morning. When he woke up he was particularly cranky and kept crying and he's been like this all day. He's also slept for most of today too which is unlike him, he's only woken up for feeds and every time he wakes up he cries and kicks his legs. I fear he's constipated maybe? What can I do to help him, nothing is settling him. I don't know what's going on with him.