Question: I'm currently playing a piece which requires big stretches of large intervals across the keyboard. It slightly hurts when I do so, and I am wondering if I should continue to practise like this.

[![Bartok Suite op14 - Allegretto](https://i.stack.imgur.com/6cfvl.png)](https://i.stack.imgur.com/6cfvl.png)

My piano teacher suggests fingerings with a combination of 3 1 and 2 1 in the L.H.

Answer: 
Don't persist in doing anything that hurts. 

As commented, if you show us the passage in question we might be able to suggest a strategy.
Never do something that is causing pain. If your hand cannot do the "stretch" you may have to shift. Is the stretch for a chord or a single note run? 

I would like to add to this that in general it is a good ideal to stretch before playing as a warm up. But never to the point of pain. We all have to deal wit our individual physical limits. Perhaps in time your body will adjust to get the reach it needs but if it's a far reach then probably not. Pain is different than discomfort. Many players experience a feeling of awkwardness as beginners but eventually get comfortable with the instrument and the demands it places on the body. But pain usually means injury and once you injure tendons, joint cartilage etc. they will scar up and that typically reduces mobility. Stretch gently as a warm up and see how it progresses. Never force you hand beyond its range of motion.