I don't know whether i should message my ex or not,i feel very upset when i see photos of him & he has been hanging out with groups of girls lately.

Count your blessings: Since he is your ex, does it mean that you don't have a loving relationship any longer? It is your chance to reconsider your situation, take stock of your qualities and move on, what he does should not concern you. Somebody better will come along and will treat you better.

No: please divert your attention away from him. Thinking about him, following him and looking at his pictures with other girls, will always make you anxious and envious. It is his own life and let him live his life the way he wants. You may use this energy to make your own life better by seeking other avenues of happiness and other relationships