Well without seeing a picture, I am not sure I could say. But if you suspect it's rotting I'd try and let it dry out and maybe consider trimming the ends that appear rotten.

Hopefully someone with more experience will come help you.

Good luck!

Ali,
I agree with Roxy that, without a picture, it is difficult to tell exactly what is happening. There are several possibilities. With the tropical downpours, if the new soil does not drain quickly, the roots may be rotting from excess water in the pot. Make sure the pot has drainage holes in the bottom and that the soil drains quickly. Another possibility is black tip fungus because you mention the tips are getting darker. Black tip causes the tips to turn black and die. If not stopped, it will kill the plant. There are sprays and systemic treatments for black tip and other fungus problems, but I don't know what may be available to you.

Check the stem. Does it feel firm, or soft? If it is soft, you may have stem rot. You could try cutting the plant back to firm wood if the soft area is well above the soil.

Hope this helps.

Good Luck.
Mike

Yup, hard to tell. I'd take it out its pot and check the roots. Are they white and healthy looking? Then repot in a better, lighter soil and keep the plant out of the rain. Make sure the pot has drainage holes. Perhaps don't put the plant in the elements until after the rainy season.

are you sure, you din't just bought just the cutting? you can try to keep the soil dry out abit, damged root during repot need to be heal in weeks before freely watering. also, try to move the plant to morning sun only to filtered shade.(sometime nusery grown the plant in shade and now you expose to full sun that will burn your plant!)