Shhambo, it's the extra carbs.
Each gram of carbs sucks up 3 to 4 grams of water, so the scale always goes up when we overindulge on carbs. But it's probably mostly water weight and will be gone within a few days.
Walking and aerobics are terrific cardio workouts but don't build muscle (only weights can build muscle), so I don't think the extra weight is muscle (it also takes a l-o-n-g time to add even a few pounds of muscle when you're lifting weights). Just get right back on track with your eating and the 3.5 pounds will be gone soon.