Get yourself to an osteopath, yesterday.
I hurt my right shoulder a few years back, similar sort of pain, just an aggravation and easy enough to work around. I believe I did it on an incline bench, but the pain itself was vague and I was never really able to pinpoint an exact location. Had some ultrasound treatment, which helped the symptoms but did nothing to address the cause or cure, however it was good enough and I was getting only a minor twinge here and there. I'm good with pain anyways.
Dec.2 while benching I felt a distinct tear take place, like an elastic had snapped in my front delt. I've had 3 treatments since then and discovered tonight from the osteo that basically my teres minor is shot. It has had so many tears that when the she started to apply friction on the extended area it felt like she was running her thumb over a bumpy dirt road. We haven't got to the supraspinitus yet, but it is pretty messed up from what I feel when she is trying to stretch out the entire area. She feels that the larger delts, traps, lats and rhomboids have been carrying the load for a long time and have had enough. The snap I felt was my front and part of the rear deltoid tearing.
Breaking up the adhesions is a sick kind of hell. Presently I can't use a mouse for more than 10 minutes without experiencing an 8 on the pain scale, which is a bitch since I am a graphic designer. This shit does not get just get better with lighter weight and some rehab, you need to get rid of the scar tissue and the longer you continue to lift without doing that, the worse it gets with continual micro-tears happening.
So even if it doesn't seem that big a deal today, if you want to keep lifting in the future, go get it treated. I am pretty much resigned to not pushing heavy for quite some time, if ever again, as I went thru a similar, although not as extensive, experience with the left side last year.