Sore neck

Hi:) i have been working out faithfully for years and every time I try to lift heavier (and not even super heavy like 15 pounds) my neck and upper back get very sore and I get a headache. My legs are quite strong and can handle heavier weights but my arms/upper body aren’t there yet. I even tried going to a personal trainer to make sure I was lifting properly etc but I was getting sore doing that too. I’m really frustrated as every time Lift heavier, I’m in agony for days after. Am I not doing enough warm up? Cool down? neck stretches? Any suggestions would be appreciated. Kind regards,

Tamsin