This sounds exactly like what mine did around 6 months. When they start realizing (become aware) that you will come when they cry, they will start to manipulate. I love kids, but that is how they are! I would say, let the baby cry it out. It is hard to do no matter what age they are, but with my experience it is much easier when they are younger. If you must go in to comfort baby, DO NOT PICK HIM/HER UP!!! Simply stand beside the crib and speak softly and rub the baby. I have found that even this has a negative impact (especially if you're child is very smart) and they know they are not going to get their way. It takes about a week before they will start sleeping or going to sleep on their own.
Another thing that helps is having a set bedtime schedule. Do the same routine every night (at the same time if possible) and they will begin to respond. Babies thrive on schedules.
Hope this helps!!!
M.