According to cognitive models, the negative perception of one’s performance and the post-event rumination (PER) occurring after stressful social events maintain social anxiety. These aspects have hardly been studied in music performance anxiety (MPA), a specific form of social anxiety. The first aim of this study was to analyze the development of negative and positive PER over two days following a soloist concert, depending on the usual MPA level. The second aim was to investigate if subjective performance quality serves as mediator between MPA and PER. Negative and positive PER were assessed 10 minutes, one day and two days after a concert in 72 music students with different levels of usual MPA. Subjective performance quality was measured 10 minutes after the study concert. An increasing usual MPA level was associated with more negative and less positive PER. Both decreased over time. Negative PER decreased less rapidly in high-anxious than in low-anxious musicians and positive PER decreased more rapidly in low-anxious than in high-anxious musicians. Subjective performance quality mediated the relationship between MPA and PER. These findings extend previous knowledge in social anxiety to the field of MPA and have implications for interventions aiming at reducing MPA.