A 2.5 lb weight gain may or may not represent an actual gain in body fat. Weight fluctuates a lot from day to day, generally based on hydration (water retention) and, to put it delicately, the contents of your digestive system at the time you get on the scale. How often are you weighing yourself? I usually recommend weighing yourself every day, first thing in the morning, naked, and log it. Ignore the number and watch for trends instead. I attached an example from my own log to the bottom of this post as an example. The scale is 1 lb, so you can see how much variation there is.

Unfortunately, at your age it's harder to build muscle and you have less lean mass (muscle, bone, other non-fat) than younger folks. It's going to be harder to lose weight because your BMR is lower than most of the advice you'll read accommodates for. Slow and steady.