hey Stew

Coming up really late to respond to this thread...the above differentials you've been given by everyone are valuable and could be the reason for your problem...

My opinion would be that you may have injured the anterior structures of the joint with that last untimed maneovre...

the long head of the biceps, the anterior joint capsule , anterior deltoid could all have some issues...
You probably get too much anterior displacement when bringing down the weight on that shoulder indicating
1) instability,
2) muscle imbalances (such as some tight structures)

You may benefit from some taping to keep the anterior shoulder more stable when you do this presses...

if you are still getting anterior shoulder pain, then there truly might be too much anterior displacement of the humeral head...

if you have to train with those weights
my advise would be to reduce the reps you do..., to probably train with lighter weights,

a good exercise wuld be to use the other had to touch the front of the shoulder while lying supne (on your back), lift your arm up, if you begin to feel too much movement to the front bring that shoulder back and continue...you can include very light weights for this maneouver and do slowly to gain control...

always train with someone to assist...so that you dont don put excessive strain on that shoulder...