Could it be that the cold attitude is just because of period cramps? Nah, no way.
Literal
Maybe prickly-being [topic-は], menstrual-pain just [casual-とか]? No, surely-not [casual-ね].
ツンツンしている ('being prickly/standoffish') uses the mimetic ツンツン — the same ツン from ツンデレ. 生理痛 ('menstrual cramps') — 生理 'menstruation' + 痛 'pain' — is stated bluntly. The speaker floats then immediately dismisses the theory with いや、まさかね ('no, surely not'). The self-correction pattern (hypothesis → denial) is very natural internal monologue.