D.
Actually- it's quite typical to see this change at 6 months. As children grow and develop we see disruptions to their sleep and eating patterns (they are learning so many new skills and growing at such a rapid pace).
here's more info on what is sometimes called the "Wonder Weeks":
We know that development in physical skills happens in 'steps' - one day a baby can take a step and start to walk, whereas the day before he could not take that step. Similarly, it is thought that the way that a baby can think, feel, notice and understand what is happening around him (mental development) might also happen in steps.
For example, at around the age of 8 weeks, a baby can begin to recognise more of the people and things that she can see around her.
At about 6 months she can start to realise that some people are around most of the time (her parents for example), while some people are strangers. Before that time she did not know that some people were strangers.
Changes in thinking can show up in changes in what a baby can do, and how she behaves.
During these times when a baby's ability to understand the world around him is changing, he may need more attention.
He may cry more, be more fussy and need more comforting. This may be due to other reasons, such as being unwell.
Some of the babies lose their appetites.
Some babies who have been sleeping well may take more time to go off to sleep and wake more often.
Some young children who have been happily exploring their world may become more clingy, and become upset much more easily.
They may show an increased need for body contact with their mothers.
Sometimes these weeks are called 'regression weeks', because a baby or young child will seem to have gone backwards in her development. In fact, she is making a big step forward, but this step is confusing to her, and she will behave more like a younger baby.
The 'sunny' weeks
When the baby has fully 'climbed' this developmental step, there may be several weeks when she is happier. She can happily try out her new skills. She may be less easily upset (less temperamental). She may allow you to be out of sight more often, and go off to sleep more easily.
Temperament
Some babies are generally easier to get on with than other babies. They tend to be happier, cry less and go to sleep more easily.
Other babies are easily upset and cry a great deal more often.
Even 'easy' babies have these fussy 'wonder' weeks. It may be easier to help them calm down when they cry than it is to calm a 'difficult' baby, but there will be times when they are more distressed than usual.
Other things that can cause a baby to be fussy
Babies can be fussy for many other reasons.
A baby who is unwell or in pain is likely to be more distressed than usual. A nappy rash, for example, is very painful, and will cause a baby to cry more and have more difficulty going to sleep and have restless sleep.
It is often thought that babies are unsettled when they are teething. Research has shown that babies are not more distressed when they re teething, but not everyone is convinced. Maybe the teething has coincided with a fussy developmental stage.
An older baby who is separated from his parent may be very distressed when she returns. This is likely to be due to separation anxiety, not 'just a phase'.
Some babies have developmental delays or major health problems, which could alter the times that they go through these mental developmental milestones.
How to help your baby during these 'wonder' weeks
During these fussy weeks, babies seem to manage the developmental changes more easily if they get extra attention, get held more often and have extra comforting when they go to sleep.
Parents need to know that there will be these more difficult times, but that there will also be times when their baby will be happier and more easy to get along with.
If parents are not able to give their baby as much attention as he needs, the baby may find this mental developmental step more difficult to manage.
Predicting the 'wonder' weeks
Since most babies follow a similar pattern, it is possible to predict approximately when a child may have a fussy ('stormy') time. Some babies may become more distressed a week or so before or after the times listed here. Some babies may become quite distressed some times, but pass easily through the predicted 'stormy' time at other stages (they may also become very distressed for quite different reasons).
In the first year of life, the stormy times seem to happen around
5 weeks
8 weeks
12 weeks
17 weeks
26 weeks (6 months)
36 weeks
44 weeks
53 weeks (around 12 months).
There is more information about each 'wonder week' - about what development stage may be happening during a 'wonder week' in the book by Vanderrijt, H. & Plooij, F. (2003)
The Wonder Weeks.