Probably a deep muscle strain. Source: Myself, same type of injury, same symptoms. I was slammed (think mixed martial arts) while in the Marines and it wrecked the left side of my back. The military wasn't cooperative when I was trying to get it documented as service-related, so I suffered with it for the rest of my time in, ~4.5 more years.

My new job pays for my health-care coverage and so I went to see a physical therapist for a bit; here is what I learned:

1. Find a good one.
2. Do the exercises. They say 2-3 times a day, they mean 2-3 times a day. I really find it easy to do them FIRST-thing in the morning, right after work/before dinner, and then right before bed.

In terms of some very specific exercise types to look into:
Lots of thoracic stretching, and also the stretch/strengthening exercise where you pinch your shoulder-blades with your back against the wall, and move your arms up and down slowly. (If you're doing it right, you actually can't move your arms up very much at all, or will need to pinch at the bottom, decrease your pinch, move your arms up some (again, not much), then re-pinch at the top of the exercise.

Stability ball exercises with dumb-bells, seated presses on the stability ball as opposed to seated statically, T and Y's on/off stability balls. (Basically you do a plank off the stability ball, and then with EXTREMELY LIGHT (3lb) dumbbells, raise them out to the side making a T-shape as one exercise, then out to the front making a y-shape as another. (I personally think it's the t's and y's that really helped me the most). I would also do these from the traditional standing position.

Buy a speed bag (as in boxing) if you have space for one as one of the best warm-up exercise machines they had me do before doing the rehab was VERY similar to using a speed-bag. We would warm-up with that rythim of 1 minute in one direction, one minute in reverse, 4 minutes total.

We also did standing rows with exercise bands, rowing back to a pinch in the back, then rowing 'out'.

They also had me moving around just to get some movement into my body; since going to PT I try and move every-day. It hasn't completely gone away but I'd say it's about 98% better, and if you feel like I do, you'd probably say it's a success, I just have a higher-level of ok because of the limits I want to push myself as a martial artist.

Seriously, go to a good Physical Therapist, it has changed my life. It's expensive, but oh so worth it.

I should add that during the 4 years I was in the service I also tried things like regular weight-lifting and exercising programs but I was never applying the amount of work the PT did for my very localized problem. I also will say I did yoga, and even though I love yoga to the point of wanting to become a teacher in the future if the opportunity presents itself, and while I believe yoga is amazing for you mentally, and has amazing physical benefits, it did not help with this specific problem either, again because it is so deep in the back, it just couldn't work out the area as-well as was needed. But if you can afford it, go to yoga too; it gave me a peace of mind that I can only say was matched when I found the first person I 'truly' loved.(Yoga also did wonders for my breathing during jiu-jitsu, so there's that which is nice.)

Oh, and NO LAPTOP IN BED (my extremely bad habit).