RICE (Rest Ice Compression Elevation) for the first 48 hours minimum. Don't strain/stress the area again until you see a doctor or the pain subsides.
About 3 weeks ago while doing safety squat bar lunges I felt/hear a loud pop near my right hip/upper quad. It hurt like hell and I immediately walked the bar back to the rack. I thought my hip was dislocated or I'd torn my quad near the hip flexor. I quit the workout, iced it every 1.5-2 hours for 20-25 minutes while keeping it elevated. When not icing it I wrapped it w/ an ace bandage for compression, which made it more comfortable. It hurt when walking up stairs, sitting down/standing up, and even while walking. I also took motrin (800mg advil) 3x per day w/ food.
2 days later I saw an ortho, luckily the injury hadn't swollen up or bruised at all, and the pain had gone down some. He diagnosed it as a grade I muscle strain. Two days later the pain was barely noticable. I waited an extra week before doing squats or anything that incorporates quads (I still worked glutes/hams/low back/calves) and then went back to 100% and was fine, that is, until I re-injured a fixed hernia op on monday of last week. Now I'm out for 3 months minimum
Anyway--just ice it, rest, SEE A DOCTOR, rest, don't stress and keep us updated. Don't stretch it yet until you know what it is. If it is strained badly or something along those lines, stretching may not be a good idea. Find a good ortho and follow his advice.
HTH
EDIT: Sounds like it could be either a lat or shoulder injury after re-reading your post. Where is the pain concentrated, in your armpit area, upper back, or deep inside the delt? Could be a shoulder injury (pulldowns/pullups case a lot of internal rotation and put more stress on the small inner muscles of the delts than rowing). Does rowing w/ dumbells, low cable, chest supported row, etc. cause this discomfort?