Skin of under arms /inner thighs darken frequently due to a condition called acanthosis nigricans
. This is frequently associated with obesity/ thyroid
abnormalities/family histories.The first treatment of this is to reduce weight.However, since you are complaining of dizziness/loss of appetite/lumps ,it is mandatory to do a blood work done before you start weight reduction.Only lightening creams will not help it lighten. Once you know that everything is fine ,you can start taking chemical peels
for lightening of the areas,apply creams with kojic acid
for few weeks. Do not use any depilatory creams/agents/ deodorants in the underarm areas at all!