What Causes Weakness In The Arms Post A Long Run?

Hi, yes please. I just went out for an hour long run. When I finished, all of a sudden my arms felt really weak. The inside of my forearms in particular. Like I had just worked them out extremely hard after never having worked them out before. The weakness now seems to have gone away, but I just found it strange.
Is this anything?
This probably has nothing to do with this but I have noticed recently that when I wear bras or compression bra tops, that it is very uncomfortable. So much so, that I have to loosen the garment quickly. This is kind of new to me. I feel like I used to have this many years back but, I have been totally fine in this regard.
Does any of this mean anything?

Thank you so much for your help