Baby Screaming at Night

My LO is almost 5 months old and I thought we were creeping out of his sleep regression when suddenly he started waking up multiple times a night screaming and arching his back. He squirms like he can't get comfortable. I bring him into my bed and he doesn't even want to nurse. You either have to walk him or wait out his cries until he calms down. Any ideas why this may be? His last feeding is usually at 10pm 4-5 oz of formula and he wakes up to eat again around 6am but in between then I breastfeed when he wakes up. It's been several nights now and I'm scared he might be in pain or something. Any ideas?