。、。
She's been on a diet for the past two months — the reason being that she gained way too much weight over the winter.
Literal
She [topic-は] these two-months-period diet [object-を] is-doing. [The-reason-being-というのも], winter [during-の間] body-weight [subject-が] considerably increased-too-much [because-から] [copula-だ].
Two structural pieces of interest. First, ここ２ヶ月間 ('these past two months') uses the demonstrative ここ in a non-spatial sense — 'here in time, the recent stretch' — this idiom is how Japanese says 'the last X (period)' without needing 過去 or 最近. Second, というのも ~ からだ is a deliberate two-sentence framing: the first sentence asserts something, then a follow-up sentence opening with というのも ('the reason being...') and closing with ~からだ supplies the reason. Splitting it across two sentences gives the explanation more weight than just tacking on a ~から clause. かなり ('considerably') intensifies, and ~すぎる ('too much') stacks on top, signaling that the speaker views the gain as clearly excessive.