Screaming/Crying

My little boy is 9 months old and the constant upset is really getting to me and becoming a genuine concern. He can go from laughing and smiling to being inconsolable in 0.2 seconds. I usually put it down to him being tired but he will not nap during the day without a meltdown beforehand and it’s making every day miserable for us both; he screams, cries, arches his back, rolls on his front, sits up and restrains against me whenever I try to get him to nap. I’ll eventually get him down but it’s an absolute ordeal. However, even when he’s napped, fed, changed, etc he’ll still cry. For example, he’ll be happily playing and giggling at me then just start scream crying. It’s quite easy to stop but it’ll start again just as quickly. Any idea what this could be? He’s a very happy, smiley baby and developing really well (stands himself up, has 8 teeth, sits up, says “mama”, eats well, drinks well etc) - I’m just wondering if anyone else’s Jan baby is the same without any obvious reason what’s wrong? He keeps pulling himself up on things and standing up. Could it simply be the frustration of wanting to get walking? Or teething still? Or perhaps it is just tiredness?! He also refuses milk now during the day so I’m also thinking it could be hunger?! But like I said - even when all the boxes are ticked, he’ll still get grumpy. Please let me know I’m not alone! 😂