My son is 3 weeks old, breastfeed him. today he is spitting after each feeding & his motion is watery.any serious? can breastmilk soured inside breast?

Normal ; no: Breast milk cannot sour inside the breast. It is the perfect storage container. You likely are starting to produce more milk, and he is overfeeding himself a little bit, causing the spitting up. Baby poop at 3 weeks of age is supposed to be very runny/watery, where it soaks right in to the diaper, so this sounds normal as well. Make sure you are burping well every 5-10 minutes and between breasts.

Spitting up & diarrhea: as long as it is just a little spitting up, no vomiting of whole feeding after each feeding, no projectile vomiting, the stool has some seedy solid particles, baby is playful & gaining weight. It should be ok. Otherwise take him to his pediatrician. Enjoy your bundle of joy. Good luck