It wouldn't have surprised me so much that a change in ammo (especially if it involved a significant change in bullet weight) would have changed the point of impact, though I would still expect the newer ammo to group, even if in a vastly different spot. Since the barrel is prone to flexing upon firing, even a relatively small change in velocity or bullet weight could make a significant change in POI far greater than might be suggested by the balistics tables.
A couple more rounds, taken slowly with time for the rifle to cool would have been helpful, at least to determine whether the distance between the first and second shot was user error, though I think that you were probably correct that it's hitting at a different spot.
Could you tell us what type of ammo it is? Since the Mosins usually have more generous bores (i.e. .311 or .312") it's possible that an otherwise good rifle might not shoot well with commercial rounds or even reloads with narrower bullets (i.e. My Lee reloading manual lists .308 bullets for 7.62x54R, though I'm pretty sure that I'd probably want .310 or .311 bullets for most rifles, perhaps with the exception of some of the Finn rebuilds).
I'm also curious as to whether you changed your shooting stance or position when shooting off of your truck. The normal preference with a smoother bolt gun is to snugly rest the forened and possibly even the but into a position that won't change at least while firing a single group, since once you adjust the rifle or move your hands, shoulder or cheek, that could greatly effect your group size and POI. Easy enough in theory and practice with a smooth K31, Mauser and perhaps even Enfields, but the stiffer bolt of the Mosin usually results in some movement. I've also once or twice found that I shot high when shooting impromptu off of a table with a lower resting spot for my rifle forend (the awkwardness of the position naturally points the muzzle upwards).
We need more details. I'm pretty sure that the more experienced milsurp afficionadoes could better help troubleshoot your scenario with more information.
Cheers,
Frank