Nightmares were my first thought aswell, only becasue we have just been through the same thing with DS 22mths.
He would wake randomly through the night screaming blue murder and the only thing that would calm him was cuddles, and then when you put him down he would start up again
They often occur at times of stress and change in a LO's life, and when I actually thoguht about it, in the space of 2-3mths, DS had had chicken pox, an eear infection that took 4 lots of antibiotics to clear, 4 canines cut through, I was sick and in hospital, he graduated from the nursery to the toddler room at daycare and then DH has been workign flat stick and had about 3 weeks where he wasn't home much. That is a lot for a little guy to deal with, so tackled the 2 things I could change in that list. Daycare-I made sure i stayed with him for a good 15min before leaving in the morning until he was settled, and DH has made a realy effort to get home earlier and spend time with him. It has been almost 2 weeks now and we have had 3 nights where he has woken and those times he has settled himself back to sleep.
Sorry for the novel, but it sounds like your LO has been through a fair bit too. Add that to being a bit OT from all the NWíngs anyway.
Good luck