It's not the fenugreek, that's just coincidental timing. Sounds like baby has a dairy sensitivity. Eliminate dairy completely from your diet and baby's poop should turn back to yellow. Takes about 2 weeks for it to leave your system and to notice the change but it makes a huge difference!
^^ this. Try making sure baby is feeding for at least 15 mins at each feed so they get the high calorie hind milk and see if that changes it. Also if you change breasts half way through a feed try to empty one breast at a time.
Re: Fenugreek changes babys poop?
1st pregnancy - total surprise
DH & I married almost 2 years