Shoulder soreness from hydration pack

Hello everyone, I've been hiking for years but recently I've moved into trail running, and I'm starting to notice a reoccurring problem. When I run I typically carry a hydration bladder with around 1500ml of water, and a few other small things so my pack weighs ~4lbs. In addition I've added a very light hip strap, to the bottom of the bladder just to keep it from bouncing around so much. All that being said, I've noticed that on the day after may run my trapezius muscle up around my neck are really sore and achy, and I'm trying to figure out the cause so I can address it.

Have any of y'all experienced this or have any tips for dealing with this pain; is this just something I've got to build up a tolerance for? It's crazy 'cause I just ran my longest trail run (7 miles) and while my legs are a little stiff, I really don't notice them because my shoulders hurt so much.

I suspect I'm not using my gear correctly (to much weight, sternum strap too tight ect.) but I can't seem to figure out what I'm doing wrong.

Thanks for the help.