I connected to a suspicious public wifi using my phone. Is it possible that my phone was hacked?
Is it possible? Sure... Is it likely, no.

Let us consider the scenarios where your phone could be hacked.


Your phone does not have the latest security updates released by google, the network you are connected is rouge, the hacker knows how to exploit the vulnerability on your phone.
You have an app installed that is malware infected, the network you are connected is rouge, hacker's network is registered with the malware infected app. He would then exploit the app to hack your phone.
Network you are connected is rouge, the hacker knows vulnerability related to android OS or a popular android app(that is installed on your phone and) that is not yet fixed by google/your phone maker (for your android phone) or the developer of the app(in case of the vulnerable app).


As you can see, there are multiple parameters/conditions involved here, I believe it would be a rare case where your phone is hacked. To answer your question, as @acejavelin commented, 


  Is it possible? Sure... Is it likely, no.