The first time it happened I was just viewing the perks to plan what to level later. When I closed the screen it suddenly said I was carrying too much. So when I checked, I noticed my max had gone down from 320 to 295. I could not for the life of me figure out why so I reset to a save 2 hours earlier and it sure enough, it was 320. I replayed most of the same parts except killing Swan again and then fast traveled home and suddenly it happened again. It went down to 295. The only thing I can figure is I have the Strength perk for +25 and for some reason it stopped working.

Is this a glitch or am I missing something here? For 10 hours or so it has been 320 and now it has dropped twice to 295 for no apparent reason.

Do you have any correct poisons or alcohol that is bringing it down?

No. The first time it went down after closing the Pipboy and viewing the Perks. The second time it went down after loading Sanctuary after a fast travel. I also had a glitch where after I switched Dog out for Strong, strong wouldn't follow me and would randomly appear next to me in a crouched position and not shoot anything. I dismissed him and he just stayed there so I thought fast traveling would fix him and then when I arrived my capacity went down 25 again.

Load back to your save of where it says 320 and check to see if you're under any drug effects. That's what it's sounding like and they are wearing off.

Where can I view drug effects and how long would they last? I thought I had been at 320 for hours.

It can also happen if you have perk "Lone Wanderer", which will boost your carry capacity as long as you don't have human companion. If you take human companion, perk will no longer add to your carry weight.

Second thing - you can have armor with pockets (it will also add your capacity). If you take it down, exactly this can happen.

Go to STAT in pipboy and if you're under the effects of a drug you will see an icon on the left, also you can press R1 to 'Show effects' in the same page. You can also check your perks in this page by pressing the right button a couple times, maybe you have the perk mentioned?

Question ? If I have pocketed armor will my carry weight drop when I enter power armor ??? I noticed My carry weight is higher when I am not using power armor but I remember it saying power armor was supposed to increase carry capacity . . .

When in power armor stat increases from your worn armor get ignored. Power armor only increases your strength which in turn increases carrying capacity. So the strength increase your getting may be giving less of a carry weight bonus than all the pocketed things you have.

I still haven't found out the cause :/. I've put another point into the perk to add another 25 and it's back to the original 320 and so far it hasn't dropped again but I'd be lying if I said I wasn't anxious.

Theres a lot of things that can effect it in this game, the pack mule and lone wanderer perks being the most obvious ones, but also crippled legs and power armor can change it too.