My 23 month old son is clenching his fist when frustrated and trying with all his might like you would when you cant get a bottle of jam open. We just weaned the pacifier so I am not sure if he is just frustrated with out it. It seems to be in the morning or at night.
That doesn't sound like anything to be concerned about. Kids get frustrated, and at 23 months he can't really express with words what he's mad about, so he clenches his fists. It's when he starts to hit people or walls with the clenched fist that you need to put a stop to it. Now, you can help him calm down by grabbing his hands, look him in the eyes(get eye level on the floor with him)and say,"I see you are having trouble with (such and such)let me help you fix it. But beware, you will then become "mommy fix it." Don't worry too much, just guide him. I have a boy and a girl, and I have learned that boys are very different from girls in every way. They play differently, talk differently and show frustration differently. It is our job as moms to help our kids figure out why they are frustrated, and then show them the way to making it better.
It is normal. I wouldn't worry too much about it. Just try and don't pay a lot of attention to it.
I wouldn't blame the pacifier at all. Some kids throw toys or cry out of frustration, until they have to words to talk to you about what is bothering them it is normal.
As long as he isn't hurting anyone with the fists or himself, I would just ignore it.