Ehehe, I'm so uncoordinated that as soon as I start running even a little, my side starts hurting right away.
Literal
Ehehe, I [topic-は] exercise-dunce [copula-だから], little run-[conditional-と], immediately side [subj-が] painful become [regret-ちゃうの].
運痴 (うんち) 'exercise dunce / athletically hopeless' is a playful slang compound — 運 'exercise/luck' + 痴 'fool' (modeled on 音痴 'tone-deaf'). 脇腹 'side/flank' is specific body-part vocabulary. ~ちゃうの combines the casual ~てしまう with the feminine explanatory の. えへへへ is a shy, self-deprecating giggle.