I've been doing pike push-ups for weeks, but, for the past couple of days, I have been getting a sharp pain in my left shoulder when I do them. After checking my form, I have seen that it is proper. Why could I suddenly be having this pain in my shoulder? Thank you.
Go to therapist / doctor. If you don't want - try tests like this one - to identify your problem. Then read/watch/find out how to fix the problem. Usually doctor is much better option - like that two from video. In case of problems I would choose them, instead of trying fix something for myself.