hi charm, welcome!
i think each person so far has brought up some wonderful points... and i don't see anyone lecturing you or criticizing you - although i know i felt that way when i first started posting. as i've posted and read more here, i see that everyone was just being honest about their experiences and the alcoholic in their lives.
my ex's mood changes constantly. i was left for someone else who she had been with before and who she had secretly been talking to every day while we were together. i could feel her growing more and more distant towards me, until i eventually found out about it, and we split up. she continued to hang out and talk to both of us (stringing me along and making sure i was always there to fall back on, i presume) and when they fight or when they're having problems, she suddenly misses me again and makes contact.
your ex very well could be drinking again, or he could not. maybe the girl he met is no longer giving him what he needs, so he's turning back to you. respektingme was very right above when she spoke of how alcoholics are very self-absorbed and will control and manipulate to get what they need or want.
i was with my ex for two years and during that time, i never once saw the personality traits i see in her today... because she didn't bring them out. she didn't want me to see those parts of her... so, she became very good at hiding it and manipulating the situation. but mood swings, yes, i've seen them.
(keep posting here; you'll find it helps you a great deal!)