Right is not stiff for a right hander, it's weak. Regardless, I seriously doubt the arrows are too soft. You have a fair amount of weight up front, but 2018s spine at 67-70#
Depending on what you were shooting previously, the 2018s might be a little smaller diameter. That means they ride closer to the center of the bow. If nothing else changed, that would point them a little more to the right. The narrower diameter would also have the effect of raising the nock point a tad, because the center of the arrow would also rider closer to the shelf. That would have the effect of making the arrow shoot lower.
Both of these would be small changes though, and probably not noticeable.
Are the arrows just physically lighter than what you were shooting? Have you checked the nocks and points to make sure they're on straight. Has your brace height and nock point stayed the same?
If it's none of the above, it may be something in your form. Or, your brain may just take a while to adjust.