The AC joint is located on the top of your shoulder where the collar bone attaches onto the acromion of the shoulder blade. You can experience pain and clicking in this area when you’ve injured it. Any movements where you try to lift weights above shoulder height usually aggravates it.
Here's a video I did about this:
Why does this happen?
It is most often linked to repetitive strain from lifting very heavy weights on a regular basis – especially if you’ve ramped the weight up quite quickly. This causes the ligaments and connective tissue that stabilise the AC joint to strain.
How do you get it better?
You’ll have to dramatically reduce the load through the joint to allow the ligaments and joint to settle down. This usually means no lifting above shoulder height for anything between 4 to 6 weeks. Once you go back to lifting, you’ll have to start with very light weights and slowly ramp it up, otherwise you’ll just reinjure it.
Make sure that you also correct any issues with your technique that could have contributed to the issue.