In the last two weeks or so, my arm has been getting pretty fatiuged with in 10-30 minutes of play. Very unusual for me, I'm used to going 3-4 hours at a time. The fatigue is in the side of my upper arm when I pull the dart miliseconds before release before release. It's not necessarily affecting my accuracy but it is bother some for sure. The fatigue has been the same sensation my legs give after a long run.

Lately work has picked up a bit, lots of moving boxes/pallets/etc around and stocking shelves. Probably related.

Any suggestions?

are you sure it is fatigue and not some sort of strain or injury, seems a little strange?

Maybe try some stretching exercises beforehand.

I think it sounds like you have worked some extra muscles with all the extra lifting and then not given them enough recovery time or not used them again for a few days so they are at the in-between phase. Either take a couple of days rest or do some extra press ups or dumbbell curls to get them past this middle phase. It will just go on its own but will take a week or 2 it happens to body builders a lot when we used to change our routines & train harder on body parts we neglected for a while. It will pass naturally so don't worry.
Grip might also be right and it could be a strain so again rest may be the only option, or just play for 10 mins then stop but be strict with yourself or it may take 6 to 8 weeks to heal

im not sure this will work for you and not sure on your style of throw was told years ago to warm up first 4-5 throw just throw at the bottom of board as this gives you full extension of your arm could be also your tense up just throw darts freely with out aim and as grip said some stretching befor you start i have simlar problem with elbow joint goodluck hope this helps