Causes of POI rising?

I noticed today after firing about 30-40 rounds of .45 acp at about 25 yds. that the point of impact seemed to rise a few inches.  Although I'm a new shooter I don't think it was because of me.  Is there something happening with the gun that might affect this change, e.g. heat from previous firings, powder residue, etc?

Typically shooter error! If your grip isn't consistent you will have a tendency to shoot 1 o'clock group (right handed). Basically if you relax your hand the pistol will have less resistance and recoil will cause group to shift up. A properly built 1911 .45 will not vertical string shots.
Jon

One of the most common errors for a newer shooter is breaking the wrist down at the moment of pulling the trigger.  Perhaps you are holding the muzzle more still and your new POI is where you should set your sights.

Shorter, stiffer barrels and other parts, no stock to pressure bed or free float from, and much less bullet time in the bore makes high-accuracy pistol work far less complicated than chasing the accuracy demon with a rifle.

IMO though, it's harder to make truly amazing ammo for pistol.

It's almost equally harder to shoot the handguns truly well.

So yeah, a relaxing grip as the session progresses is the first variable to investigate. For me at 25 yards with the 1911, relaxed grip can move the group center from the X-ring* to the middle of the 9 ring. For me both are centered.

*That's the group CENTER, not the group itself...