I have faced before the same. This happens if you add more chemical compounds (e.g. NaHCO3 or something else: check what you added) than you are supposed to add. So pls check what did you add and reduced them accordingly. 

Dear jamail !! 

Thanks for your answer . 

I just seed my MSCs in the flask T 175 and i add media 35 ml , media alfa mem ( sigma M6074) + 1 % antimycotic /antibiotic   + 10 % FBS . that's  all i do , i did add  anything . 

I think this happens due to pH change only 

dear arvind ,

thanks for  your answer , can you suggest  me how can i avoid this problem ??

The pH change can be avoid through use of buffer

So we have two mains buffers : HEPES and NaHCO2 ...WHICH ONE DO YOU SUGGEST ME??