Could be seat height- but if fitted by the shop- may not be this. Saddle I would not blame yet as it sounds as though it likes you- as you have not moaned about the butt pain.
Try getting out of the saddle on part of the climb- just to see if the pain gets worse or if it comes on later. And does the pain come anywhere else on the ride or is it only on hills?
Pics of you on the bike would help- in your normal riding position- but obviously stationary. Pics showing your leg with the cranks at lowest position and at 90deg will be necessary. Doesn't sound like a fit problem but may be.
Or it could be that the hill is too steep for you- Stop climbing the 25% ones for a while.
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How long was I in the army? Five foot seven.
Spike Milligan