I’m pregnant and sometimes I feel a dull pain in my stomach. Is this normal?
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of dull? It's dull.
To remember the reading, think about how when you're inebriated (にぶ) you need to use a dull knife. The にぶい reading talks about a thing that is dull. For のろい, which refers to a dull mind (someone who's thickheaded). Think about a Norse (のろ) God, the God of dullbrains.