Potty advice for 30 month old boy

We started potty training officially this weekend. He's done well, a few accidents but all expected. 

Last night he was at Nana's, she continued our routine but he had a few more accidents due to be engrossed in his toys I think. He woke up at 1am and that's him been up and awake the rest of the night! He mentioned needing a wee during the night and my mum told him he had a nappy on (probably not the right thing to say) but he's then complained of a sore tummy and back on and off since waking up. I'm thinking this really unsettled night is perhaps down to starting the potty training but friends I've spoken to haven't experienced this....could it be linked or his way of saying he's not ready yet? Any advice gratefully received thank you