I just saw my clinic nurse about this with DD. She is the same age as your little one. Apparently it is very common around this age and usually due to separation anxiety and the massive developmental leaps they are having atm. Like appletree said, it does pass. DD did this for about 3 weeks with some ok nights and some really bad ones, however the last 3/4 nights have been much better. I am not big on the whole controlled crying thing but i had gotten to the point of enough is enough about 4 nights ago and i let her cry for about 45 min (wasn't screaming, just continuous whinging) and she eventually feel asleep. From that night she has not really done it again. I think it was partly habit that she had formed as i would be going in there when she started up in the previous weeks but like i said, after leaving her that one night she did break the cycle and hasn't really done it since...I found that when i went in to tend to her she was getting even more worked up and i just thought it wasn't working me or DH going into her, just seemed to make the whole situation worse and it took forever for her to calm down.

not sure if that helps, just my experience and reassurance that your not alone! xo