At 6w, the very common deduction would be colic or reflux. Avoid any colic causing diet like cabbage etc. You can Google for a list of them. Burp your bub after each feed. You can also offer baby probiotics like BioGaia (approx $45 from pharmacies). Studies have shown a positive relationship between probiotics and colic, but no in-depth studies have explained why.
Another possibility may be sensitivity to your diet. But if your family does not have any food allergies history, you need not worry about it now.
Read more