Pip is understandably upset--since they were children, Pip has loved Estella and wanted to impress her. And, when he received his inheritance, he assumed that is was from Miss Havisham, for the intentions of bettering his place in society so that he can be worthy of Estella, his hopes soar even higher. He thinks that the entire purpose of his money is so that he can marry Estella with no sense of shame.
So, not only is he upset that she is getting married--not to him, but he also doesn't like the guy that she gets married to, Bentley Drummle. When she tells him, he even calls Bentley a "mean brute, such a stupid brute" right to her face. As it turns out, he is right--Estella does not have a happy marriage, and Bentley treats her horribly. The news of her engagement to him throws Pip into a serious depression. He is devastated, and after proclaiming in a very moving speech how much he loves her, he leaves and wanders about in a daze for hours. Her marriage is news that not only shatters his hopes for marrying her, but also the hope that he was chosen as her future spouse. He begins to doubt why he received his inheritance from Miss Havisham.
Overall, Estella's marriage to Drummle is very upsetting for Pip, and throws many of his assumptions into doubt. I hope that those thoughts helped; good luck!