it usually hurts from pulling or straining it.. numerous actions can cause it from lifting to overextending your arms.. In more serious cases you can have a slight tear in it (complete or almost complete tears you would know by the extreme pain!). I had a slight tear in mine, still not sure what caused it but part of it started from a reaction to an antibiotic (obviously not the normal case). They gave me anti-inflammatories and I did a round of physical therapy for mine, but I was in a lot of pain so I also went on Vicoden and had a cortisone shot. If it's minor, the best thing is to stay on the ibuprofen. If it hurts you can ice it off and on. Don't do anything excessive with using your shoulder for awhile to prevent from agravating it.