après que

The main verb (étais) is in the imparfait. It can't be the PQP, because soulager is conjugated with avoir, not être. In this case, soulagée is an adjective.

Hell'oh Raymond,

The "Après que" expression means something has occurred immediately after this. When you use the "Plus-que-parfait," it's like you tell a story. (something that occurred long before the present situation).

An example should help you:

"J'avais mangé le gâteau sur la table avant de partir au travail." (Long time ago, you did eat a cake before leaving for work)

Not equals :

"J'ai mangé le gâteau sur la table avant de partir au travail." (You just ate (like, one hour ago) the cake and then left for work)

So you cannot use the plus-que-parfait for expressing something that occurred right after you did something.

"J'étais soulagé après que tu avais réussi ton examen." sounds strange for a French speaker.

Example:

- Après que je suis arrivé, j'ai bu un café.

- Mon père était heureux après que je suis rentré à la maison.

But be reassured! Even we, French speakers, find this form strange because a few people use this kind of conjugation.