Given you said the main problem was your shoulders feeling really heavy, I'm joining those suggesting you hadn't pulled your wetsuit up enough, particularly around the shoulders.
Take your time putting it on and making sure you've pulled it up as far as possible to the top of your legs, then as far up your body as possible before putting your arms in the sleeves. Again, make sure that you pull the sleeves up as far as possible so that any 'slack' is around your shoulders.
Once it's on, it should feel a bit tight. If it doesn't, then it may be that it's too big for you.