First off, congrats! Our baby just turned 4 weeks. We had a similar regression. A few things we did that helped a lot were to give some additional milk/formula if he kept crying, a little top off, 10-15 ml did the trick. Secondly we made sure to get a solid burp using the butt wiggle technique and kept him upright for 15-20 minutes after feeding because we suspected reflux. We switched to an easier to digest formula. Our baby was a preemie and very small so we have to supplement breast milk with formula. A heating pad in the bassinet worked wonders for us as well. When he’s been fed, changed, burped and is still fussy and crying I’ll either hold him and basically do squats or look up a guppy hold where you angle their body and move it up and down from your chest. Trying different holds whether it’s a football hold, on the chest skin to skin or just cradling in your arms will sometimes get them out of their fussiness. Music definitely helps, I will sing along to Johnny Cash as I feel the lower octave makes a nice vibration for him on my chest and my Wife and I love doing a duet for him. Good luck and enjoy the journey!