bike training and technical questions

Hey everybody,

Hope race season is going well for all of you and that your staying injury free.

I have recently gotten back to training after a running injury while training for a half ironman. I was out for two and a half months. When I got back on my bike things went great and I was logging all time highs during training at only moderate exertion. I trained for most of the winter on a bike trainer before getting hurt. 

I am finding that I am having to switch gears frequently to find a comfortable cadence on slightly rolling terrain and that I am riding in a big gear more often. I have never had to do this before and don't understand why. In a recent race, I did excellent in the flats, but hills were quite difficult. I am also finding my gears skipping when shifting or not shifting until I hit a bump. 

Any advice/help?