Potty training help

My son is 21 months we've been training for about a week now. He's in regular undies and he likes it we weren't having any accidents until yesterday. Yesterday he woke up refusing to sit on the pot for any reason. When I asked him where we pee he said in pot when I asked where poo poo went he said in pot. So I set the clock and a half hour later I said Matthew lets go to the potty. He shook his head no and ran away from me into the kitchen where he stood and peed. Than he ran to me and said I peed yay!!!! Today I put him on the pot he didn't do anything said all done I put his undies back on, we came out and I started nursing his sister than he stood infront of me and peed. Ideas, words or advice would be great.