I have had a bruise-like pain in my armpit for a couple of months now. i have had an ultrasound which showed as normal. what else could this be?

Multiple causes: muscular strain, folliculitis and hidradenitis are all possibilities. Your symptoms require an examination.

Armpit pain: I assume no rash. Had trauma ? Which armpit? Did ultrasound check for vascular patency? Discuss w/ MD if this could be intercostal neuralgia(if trauma) or referred pain i.e. coming from nerve that transverses your armpit but goes somewhere else like lung, gallbladder-if on right , or breast. Would discuss additional imaging like mammogram, gallbladder ultrasound , CXR or even MRI of area .