Take a break and keep stretching, also it could be that you are over extending it. Just my two cents.
Lots of factors that contribute, but try some gentle massage of the area, review your cod liver intake and rest initially then straight back to it.
Rest. first and best tip. If it's still there, try some stuff to fix it like plant ointments. try to tape it next training or use bandages. better & longer warm up also an idea. maybe you're not really warmed up for throwing hooks or punches in general. use different methods to warm up yourself. other idea is to use a foam roll/some small balls to massage the area around the elbow, especially the triceps. lot of times elbow problems coming from the triceps muscle. could be a muscular thing there, an adherence or something. i had some minor elbow issues couple of weeks & months ago while training in the gym with weights, no problems at boxing & jj training but it shouldn't be like that. fixed it with resting, avoiding exercises where i had the pain and give it some time, using the methods above to fix it. now i am fully fit and recovered, no problems. But as i wrote, elbow issues are caused by many different things so be careful, it's no joke.
