Tempo & Pitch automatically change after audio mix down

Looking for help. I have Cubase Elements 7 . After recording few tracks , I have been doing some editing like cut & paste, as well as processing like gain and normalize etc. However, after I do the audio mix down to create MP3 file , the resulting file has a very slow tempo and pitch is also changed. Please help me on how to fix this problem .

Make sure, the soundcard-, project- and audiofile samplerates match.