My 3-week-old baby cries a lot while contracting as if he really had a stomach ache, could it be related to my milk?

Yes, it is possible that your baby's crying and discomfort could be related to your breast milk. It is important to pay attention to when these symptoms occur and if they are associated with any other signs like vomiting. If your baby seems to be behaving differently than usual, it may be a good idea to consult with a pediatrician. Colic is also a possibility, which can start as early as 3 weeks old. Colic is characterized by prolonged crying episodes, but it is a normal part of a child's development and usually does not affect their general health or weight gain. Unfortunately, there is no specific treatment for colic, but comforting your baby in your usual ways and trying gentle massage techniques may help alleviate their discomfort.