。
She felt her heart pounding.
Literal
She [topic-は] heart [subject-が] thump-thump does [nominalizer-の] [object-を] felt.
どきどき is one of the most recognizable Japanese mimetics — it captures the sound and sensation of a rapid heartbeat, whether from anxiety, excitement, fear, or romantic flutter. Combined with the perception construction ~のを感じる, it creates a vivid first-person sensory experience. The polite ました ending adds formality.