Wandering is onto it.
It sounds like a 2 fold item.
Part 1: If you're doing more lifting than cardio, you're probably going anaerobic, which will cause an increase in BG b/c you're not burning carbs anymore.
Part 2: Your liver is responding to the increase muscle activity by dumping some glyogen into blood stream, and when you stop working out, you're not giving your body a signal that you're done...so it continues to dump for a short time, thinking that "your body needs it".
If you are going to continue a similar routine w/ anaerobic activities like weights, you might try something similar to what Leah does (a small bolus to help combat, or perhaps an increased basal an hour beforehand). This will be unique to everyone though, and may vary based on how much actual aerobic vs. anaerobic activity you're doing at the gym.
Additionally, after a long aerobic activity (where the liver has been dumping glycogen to try and help), you might try drinking a glass of milk (good protein/carb mix) to try and help "flip the switch" that lets your body know it's not working out anymore since the blood will go from muscles to the stomach to help with metabolic breakdown of the food. You'll want to bolus for the milk of course. But again this is going to be a variable bolus based on the intensity/duration of the activity.
So it will take some trial/error/record-keeping to get it dialed in appropriately.