I'm up 4 pounds this week and I've been more active than ever, burning around 3500 cals on the 3 days I worked out. Because I weigh daily, I noticed a spike in my weight the day after an intense workout ( cardio+strength training). Is this normal?

Yes, this is a pretty normal thing - there are almost no end to things that can cause some water retention (hormone changes for everyone and TOM for the ladies, sodium changes in the diet, etc.,) and you are hitting on at least two more exercise related ones - water needed as part of the muscle repair mechanism and you are likely behind the curve on hydration so your body is playing catch up. To see if you are hydrating adequately during your workout, try weighing yourself immediately before and after to see if you are losing more to sweat than you are drinking during the workout (preferrably weigh naked so you aren't weighing sweaty clothes!)

Phew thanks for the feedback. The gain was quite alarming and counterintuitive. It feels like after burning a lot of calories, you should see a loss not gain.