、。
Even though I just had to nod politely, I ended up weirdly stiff and formal.
Literal
Just bow do [only-だけなのに], strangely lobster-fin stiff-[regrettably-てしまった].
会釈 ('slight bow, nod') is a very specific Japanese politeness term — the small, quick head-nod used as a casual acknowledgement (lighter than a full bow). しゃちほこばる ('to become rigidly stiff, to tense up formally') is a quirky verb derived from しゃちほこ (the dolphin-like ornaments on castle roofs), meaning 'to stiffen up like a shachihoko.' It describes excessive formality or nervous posture. 妙に ('strangely, oddly') modifies adverbially. ~てしまった ('ended up') marks the unintended outcome.