Been having sharp pain on top of my rib cage , just below my armpits . what can it be ??

That's often tender: area normally. The teres major and minor (side muscles of the chest) are normally tender by the ribs, and is not a problem. If your pectoralis major, minor (breast muscles) are tight, these become even more pronounced. Usually, we are not pressing ourselves there much, so it's not noticeable or bothersome. If it's PAINFUL, do see a doctor soon!