My 2 year old cat keeps throwing up help!?

My cat used to throw up once in a blue moon, but I used to think it was just hairballs making him do it. Recently he has been throwing up a lot, but there are a couple of reasons it could be and im not sure which. I changed his food from whiskas dry to whiskas dry indoor food he's ate 3/4 of the bag and still eats the same amount. Also I got a new kitten in the last two weeks and they haven't got used to eachother yet but they are getting along better than they did at the start could that be the reason. Please help!