Is it possible for my baby to be overweight?

Yes. I have seen six month old babies who are obese - if mom responds every time a baby cries, with a bottle, The baby can become obese. Have your doctor check the height and weight for age, and calculate how many calories the baby needs and is getting.
Yes. Of course! Talk this over with your doctor and look over your babies growth curve. Growth at this age is as much about speed of growth as the actually numbers. If baby is gaining weight at the right speed and the weight is isimilar in percentile to the height, then you are likely fine. But if the weight is going up to fast, or crossing percentiles, then there may be a problem.
Yes. Your doctor will check your baby's weight to length ratio and determine if your baby is overweight. To prevent excess weight gain, make sure your baby is truly hungry by offering a pacifier or other comfort when they are crying. If they refuse to be comforted then they are really hungry.