I've never heard of a "toxicity build up". A flare up "acute on chronic" can happen. The irritation and mechanical disadvantage beause part of the Rotator Cuff is not working properly can cause increased pain within a short time. Likely the work load you have wanted your shoulder to do exceeded the work load that the problem shoulder can do.......Inflammation can start in various structures in the shoulder. You don't mention who has diagnosed you/ which investigations you have had/ what was the Physio trying to massage away actually? (I sometimes use massage on trigger points around the shoulder muscles and soreness caused by movement disfunction but it's only part of the story) A Roator cuff tear is something you can aggravate with everyday activities, never mind the football bit. Try and see good shoulder specialist, get a good assessment, and maybe you need some medication to calm it down for a while. And maybe a tear has got worse......