It feels like a bruise in my biceps muscle underneath my arm halfway between the armpit and elbow. When I press it, it feels like a small lump.

Maybe. NO history is available but this may be a bruise of sorts. There may be a small vessel rupture and could have created a small lump called a hematoma. Have you looked to see if it appears bruised? Also what have you been doing ?Have you been playing hard or roughhousing. See your doc and see if he can help you with this.