I would bathe the area with hot salty water on a regular basis and still apply the germolene. If it doesn't clear up or gets worse you might need antibiotics - could be an absess. Quite common to get nasty spots or absesses where there are hair follicles and in a sweaty area. Painful though.
could be a boil...go to dr to check it out in case infected (as can happen/ blocked sweat gland) and you need antibiotics and obviously to rule out anything else. Boils often happen when immune system low so maybe a tonic/well-balanced diet. keep area clean -salt baths- and dry carefully. Important to keep dry as infection breeds in moisture.
or it could just be a spot under the arm - why not wait a while and see what happens - it may well turn out to be what sqad or LL says but it may well turn out to be absolutely nothing. - relax and forget about it for a while - the more you focus on something the worse it gets.
you are all right!! a boil is 'infected'. however it's usually caused by the normal flora on your skin. your doctor won't always prescribe antibiotics for this only if recurrent or widespread