Sore lump inside of arm, lower bicep?

It's hard to describe where it's located, but I'll try. If you hold your right arm out straight, touch the inside of your elbow joint. Now move your finger down to the meaty lower muscle (about an inch and a half or so). Now move your finger towards your body another inch and a half. That's where the lump is. It feels grape sized & somewhat hard, but nowhere near bone-like. Close to avocado hard.

I just noticed it sitting here at my desk at work. My arm has been sore for about 2 days, but I figured it was because of a way I was sitting while I was playing a video game a few days ago. To summarize, I was needing to type something in w/ my keyboard, which is on the floor (PS3). So I'd get up & crouch down w/ my knees tucked up into my armpits. I think I took too long in that position because the next day one of my armpits were a little sore (not anymore) and so was my elbow. The soreness in my elbow has persisted & I touched it today & noticed the lump.

Is this something I should be afraid of or is it just some kind of sore?

NOTE: my mother-in-law is a retired nurse, so I'm going to ask her when I get home to come over & take a look at it.