Why is my lemon tree seedling sad? :(

Hi guys,

Had a lemon tree seedling that was perfectly happy up until about a few weeks ago when the leaves started yellowing, and it now looks very unhealthy, the stem is still green but it hasn't grown for over a month and all the leaves look like they will fall off.. It usually stays in the bathroom but the lighting isn't best there and the ill health started to appear when I tried moving it under my 400w LED array to see if the light would help it to grow. I'm not sure what to do now, pictures included.

 

I'd be interested to know where you got your lemon seedling from?
To me it looks more like Poncirus trifoliata, which is often used as a root stock for growing citrus in pots, in cooler climates.

I doubt its actually a lemon seedling.