Water retention or weight gain?

So, I guess this is more of a two-part question.

I need some guidance on health and nutrition. I use Beachbody workout programs like 21DF, but I've encountered a bit of an issue.

Lately I've had some health issues because of personal things going on in my life. I have over-exercised and my nutrition has not been its best, and I'm already a slim frame. I'm 5'4 and about 5-10 pounds underweight now (oops!). So, I'm trying to put on healthy weight. I know how to do it--cut out some cardio, up the strength training, and eat super-dense nutrition and lots of it.

But, for the past week or so my diet has been crap. I've been gorging myself (quite literally--I can tell I haven't taken a cheat meal in a while because I've been eating TONS of junk food) on Mexican food and tour of Italies at the Olive Garden. I've been eating pretty badly over the past week and the scale shot up about 5 pounds. But I've kept working out every day by doing tons of cardio (I usually do HIIT, running a ton, etc. My exercise regimen over the past couple of weeks has been only intense cardio). My first thought is "yay! I've gained five pounds!" But how much of that is true weight gain do you think?

I can't tell if I've gained at all, but it seems to me that gaining five pounds in a little over one week is impossible with the amount of cardio I've been doing (when I say I've been doing quite a bit, I mean it). Any advice on all this would be appreciated.

Also, I know how to gain the weight healthily: cut down on the cardio, focus more on strength training so the weight gain is lean muscle, and eat more nutritiously and lots of it. I guess I'm just curious to know where I stand. Are those really five "sticky" pounds, or is it impossible to gain weight with the amount of cardio I've been doing? (When I say cardio, I mean a long hour of running fast on the treadmill, a hIIT workout, lots of tuck jumps, etc.).