yes it could well be hacked
I'm no expert on phones, but hijacking malware exists for all flavours of smartphones: Windows, Android, Apple
You don't say what kind of phone or operating system you have, but the basic principle is reset it to factory defaults, wipe/format any memory cards and start again from scratch - and make sure you use security software
Change the password on the e-mail account, and also change the password / security recovery questions (the last bit is important - if someone has hacked the password, they may have access to the recovery questions and may be able to hack the account again after you claim it back. You MUST change that at the same time as the password)