You did not gain 5 lbs of muscle a week eating 3500kcal/day.
Especially if that’s you in your profile pic.
A lot of trainnees on their first cut/bulk cycle make the mistake of weighing themselves too early and too often. This gives you the experience Zag suggested, where the scale lies. That 5lbs could have come from water, indigested food in your gut, glycogen, or a number of other factors. It doesn’t necessarily mean anything.
When bulking, 3500 is a good starting place. Weigh yourself once a week (same time, after pissing, before breakfast, like Trenchant posted), and don’t ever freak out. If you’re gaining weight, good. If you’re not gaining weight, wait for next week to confirm it, and then bump your kcals a bit. Also keep an eye on your mirror- when you’re as lean as you are (or as your avatar is) you can tell when you’re gaining fat/too much fat. This may not be that big a deal for you. It may be an incredible deal for you. The point is to take measurements once, weekly, and adjust what you’re doing based upon what progress you make doing them.
So weigh yourself next week, and see what comes of that.