It's a coin of Constantine II as caesar, and it should be this one, though I'm not the biggest expert in this time period (photo from Wildwinds): Obverse: CONSTANTINVS IVN NOB C, laureate, draped and cuirassed bust right, seen from the front; Reverse: CAESARVM NOSTRORVM around VOT (dot) X in wreath. Mintmark (dot) AQT (dot); RIC 101
It's a votive. During this time period, Augusti and Caesars took vows of office every 5 years, thanking the gods for their protection for the previous 5 years and pledging to serve faithfully for another 5. Coins were often minted to celebrate these ceremonies, and your coin good is a nice example of this.