Hi I'm 14 years old. Yesterday i was doing 30 lb flies with a dumbbell, after i stopped working out my shoulder was hurting a bit, i wake up this morning its killing me, can someone tell me what to do please? Thank you.

I currently do 12 reps on one arm without stopping and then doing the same on the other arm using just one dumbbell. Is that fine or is it better to have two dumbbells and alternate sides after each rep?