If you feel that you're pants are tighter, you may be experiencing some bloating due to water retention. When a person works out intensely, their muscle fibers soak up water like a sponge. this is what they are supposed to do. your muscles will release any excess water they don't need once they've adapted to the new routine.
If you've been running on a regular basis, you've probably increased the lean muscle in your legs. Water retention can cause your muscles to swell. But, that swelling should go down in a few days.
Have you been logging your food choices ? Eyeballing or estimating isn't enough. If you're not tracking your food, mindless eating or the extra nibble here or there could be the cause of that five pound gain.
Could have you have gained five pounds of muscle in a year ? Assuming that you are not eating more than you think, yes, you could have gained five pounds of muscle. However, do you look more defined ? a five pound muscle gain would be noticeable. Clothes should fit a bit better, not tighter if you gained muscle.
Not to get nosy, but did you change birth control ? I know women who made no changes to their nutrition or exercise routine and gained weight when they changed BC. the weight gain was a result of water retention and bloating.
So those are a few possible theories. If you're not logging your food, I'd start by logging again to see if there has been a change to your eating habits.