Why does my arm hurt when I flex or do reps with a dumbbell?

You have to be careful with weightlifting. If you strained a muscle, you definitely need to let it rest. Wait until it doesn't hurt from normal everyday work, like lifting a backpack or a box or something. If it feels normal again, then I would start working your way back up to 35 pounds, but don't start with it. Start at 10 and as long as there is no pain, then move up to 15 or 20, then 25 then 30, then 35. Just listen to your body! If it hurts, stop immediatly. If you don't let your muscles heal properly, it could hurt you in the long run. Like, you may not be able to workout that arm ever again! If it keeps hurting, you need to go to the doctor. You'll probably be fine if you just let it rest. Remember to let your body rest. That is JUST as important as the actual workout! Good luck!