Pinching feeling in my tricep when doing dumbbell curls

[MuscleGainer]

Today I was doing my upper body workout and when I was on my 4th rep, I felt a pinching type pain on my tricep at the top of the movement where I flex the bicep. The last time I had this kind of pain before today was 1 month ago. Also this is my second workout using 40lb dumbbells for alternateing dumbbell curls and last workout I didn't have that pain and could do 5 reps. I talked to doctor about it and he said that I stretched my tricep and if it's painful stop doing it. I stopped doing that exercise right after I felt that pinching feeling in my tricep. This is only my second workout with 40lb dumbbells for this exercise. I think I worked my biceps pretty hard today anyways even though I couldn't complete my set of dumbbell curls as I was doing heavy dumbbell rows earlier in my workout. Anyone here experience this kind of pain in a some kind of bicep curling exercise? Also why did I feel pain in my tricep when that is a bicep exercise?