Hard for me to tell---as usual....Without inscriptions, I'm just guessing and my memory isn't what it once was... But I have a Diadumenian with the same reverse ---obviously also from Mesopotamia so I'll post it for comparison: I'd look for other emperors of that particular time frame for comparison as well....
Your Diadumenian has three stars while the OP coin only one. The portraits are very different as is the moon on the reverse. Odd looking coin (the OP caoin, not yours Mikey)
My gut says Caracalla. What's the diameter and weight? close example: 14-15 mm / 2,50 g Obv.: AVT ANT - ω[NЄI Rev.: KAP K - [M]H BMC 14?