There could be tightness in the shoulder you can address with many videos on the MWOD site, you can get a 10 day trial to look at them if you aren’t a member. I would start with videos that address the biceps, pec wall and internal rotation. It is likely and muscle or capsular issue given it only hurt under load. You may also have technical errors in your push up. You can arrange for a virtual consult with anyone on our staff to really solve the problem, our emails are in our bios on the website.