D.M.
What this sounds like to me, is that YES your son is becoming more active and this is affecting his sleep. Kids at this age, become more aware of their surroundings due to a development of ability to move around and discover their world.
What I would first do, is see if he will put himself back to sleep. If he is genuinely crying and freaking out and needs something, then by all means go to him. But, infants often at this stage can wake up and put themselves back to sleep with no problem if they are left alone to do so. Sometimes seeing us only make them wake up more, and then makes the process of going back to sleep longer.
If he really needs you, then you can only do what you're doing and ease him back into her 'normal' routine. Does he usually fall asleep on his own? Or is he aided to sleep with feeding, rocking, etc?
My son and I co-sleep, and I found that from 12-16 months he had various periods of waking where if I sat up and tried to get him back to sleep he would then become 'awake' and want to play with me and see what was happening. If I let him sit up and look around, he would lay back down and go to sleep. If he cried and needed a diaper change, water or something else then I'd take care of business and rock him back to sleep and put him back in bed.
Also, SEPERATION ANXIETY can develop at many ages and stages, and he may be seeking out your attention to make sure you are still there. When kids become more aware of their surroundings they also begin to understand that Mommy leaves, and will come back if I need her but oftent his lesson is learned from practice. My son would cry for me in the night, and for a few weeks I had to cuddle him and explain to him 'Mommy was here for him if he needed me' and he'd drift back to sleep. It's just a matter of creating that sense of security that even though Mommy is gone from sight, she's always there.
While, nightmare might be the issue at hand...which I think every kiddo deals with at some point. In this case, you just need to keep doing what you're doing and help ease him past the fears and anxiety that nightmares create. This too shall pass!
These are ALL very NORMAL and with time they can be worked through, but it takes time and again some TLC.
Best of luck with your little guy.