You mentioned that you tensed up which will increase the felt recoil. Think of getting a shot in the arm. It hurts much less if the arm is relaxed vs. the arem being tight and tense. This is one reason why some kids scream when they get a shot.
It could be that you do not have the butt of the rifle on the right place of your shoulder and that it was not snug against the shoulder. When a shooter is uncomfortable shooting a firearm, they tend to tense up and also try to put distance between themselves and the firearm. The result is the butt typically is not tight against the shoulder. They also end up fighting the recoil by trying to anticipate when it is going to come.
It is also possible you were not using proper form, posture and balance.
Any of the above has the potential to increase the felt recoil.