Mustard seeds for tempering lentils brought out a little bitter after taste
There's quite some bitter components in there:


Fenugreek
Nigella seeds 
Mustard seeds


In general to balance bitterness, you either add salt, fat, or sugar. 

In yours case, maybe the addition of the mustard seeds pushed the bitterness just beyond what you like. So quite possibly you could also choose to reduce the amount of nigella seeds and fenugreek.