I'm surprised to hear that you have shoulder mobility problems considering you have such impeccable form on snatch and overhead squats. Your external rotation is freakish. Craig Buker (Buk) is the only other person I've seen with that kind of flexibility.
I can't really tell being that I'm not an expert in this field, nor can I see you point to where the pain is, but it sounds like you have stretched or torn your
infraspinatis or maybe your
teres minor or
major.
3 months with no change after rest definitely has me concerned... I would go to an orthopod for sure. Hate to give such generic advice, but you're at that stage mein freund.