Any idea what could be wrong…? :(

My 10 month old is soo upset and has been up all last night, all throughout naps and tonight has been up to. He is crying for long bouts- not like him at all. Last night I went to cave in for the first time in ages and let him sleep on me whilst I stay awake but he continued to cry. Given him pain relief which seems to help but not for long.

He has no temp, and no other obvious symptoms. He had one diarrhoea nappy. Rest normal. Rubbish appetite except for breast milk. Seems a bit Weaker when walking etc. we are seeing doctor tomororw but does anyone have any idea what could be wrong? My mind is racing it’s horrible seeing him so upset