I have a skin condition underneath my armpits, started off as a rash(non ichy) then turned my skin a darker colour, and dried it up, so its choppy.

Hyperpigmentation: What you are describing is a common phenomenon known as post-inflammatory hyperpigmentation. Any time there is inflammation of the skin, whether due to a rash or a pimple or a trauma, one can then get a residual reminder of that inflammation with resulting temporary darkening of the skin. Over time, this will fade but it may take weeks to months to do so.