Does it cause pain?
It may cause some discomfort, like a slight burning or tingling feeling when it is expressed, or "squeezed out".
Will it stop, or go away?
Not likely. While on HRT you are deliberately increasing your estrogen levels, and as the body regulates your hormone levels you may notice an increase, or decrease of production of the substance; but it will not stop producing while you are on HRT.
Are there any negative effects?
The most noticable effects are in the size, and tenderness around the nipple area, which may appear larger than usual. You may also notice the forming of small pimple-like lumps; these are all normal effects, and are present in cisgendered women.
Do I need to be worried about lactating?
No. This is a completely normal part of being a woman.