| If you say that it's bursitis, I'll take your word for it. However, if you are unsure, I'd recommend Googling "rotator cuff injury" and taking a look at some of the exercies recommended for strengthening it. You mention that you were doing some kind of "press" which, if done poorly, can easily damage the rotator cuff. Bench press-type movements on a flat bench are a common source of this type of injury. I had this myself. The key is to find alternatives to the movements (use dumbells instead of a bar, etc.) that aggravate the inury and incorporate regular targeted exercises to strengthen the area.
The "pop" that you heard may have been the sound of your muscle moving over the joint due to misalignment. However, if it is anything more than that, you should make an appointment with an orthopedic surgeon for an evaluation. | 
