Assholes!
The answer is simple, Tape - just walk it off 
Haha seriously, it does sound like an impingement, as opposed to a rotator cuff or deltoid muscle injury. You should go see a Physio, who'll probably make you do stuff similar to what the Showstoppa has mentioned, or hold a tennis ball below your armpit, and have you pull the elbow towards your body with the other hand.
A Physiotherapist is your best option.
Ya, when I did it I had my arm up and rotated inward (setting a pin into the 14th hole on the rack), so perfect posture for an impingement. What's got me worried is why it didn't just hurt while impinged and then go back to normal.
Getting an xray and ultrasound done in about an hour, then seeing a gp for a referral to an orthopedist (on wed) this afternoon, then stepping next door to a physio. The pain is very similar to what I've had on and off in the past (although this might be more to the anterior side), so I figure I'll get to the bottom of it once and for all.
You guys are leaving out the other possibility of a labral tear..separate from rotator cuff.
Hoping not. Didn't happen under load or from slamming into anything, so hoping nothing is cracked in half. We'll see what the ultrasound shows up, if anything.