My mother says you shouldn't exercise too much right after lunch.

Literal

Mother [topic-は] lunch-after immediately exercise do-[excess-すぎ]-[must-not-てはいけない] [quotative-と] says

運動しすぎてはいけない = 'must not exercise too much' — ~すぎる (excess) + てはいけない (prohibition). と言う in the present tense reads as a habitual 'says,' presenting the mother's advice as something she repeats regularly.