Hi, and welcome to ATP
What you have there is a tree that was growing more slowly because of winter temperatures. You probably should have eased back on watering it through the winter. Even though it's indoors all the time, it's surely colder at night in winter than summer and definitely getting less intense light, too. That will make your tree want to slow down and maybe even go dormant.
The fact that you have new leaves growing is encouraging. I'd say you may want to re-pot it into a slightly larger pot, give it a light dose of timed release fertilizer (or use potting soil with fertilizer in it) and bring it towards the window again as the weather warms up. Then let us see a picture of how it's doing in, say May or so?
