The tension you felt is your rotator cuff being worked... very normal, don't over do it.
Def stretch it out.. 19 year old females shouldn't have super tight external rotation in my opinion.
Also stretch your pecs, getting your hands held behind your back with your arms straight is really more of a pec stretch than an exercise in external rotation.
Keep working the external rotation exercises, start with minimal weights as the muscles are only small... concentrate on form - only pivoting from the shoulder and not letting the upper arm move from the axis it is working from.

good luck,
pudding