Um...I go up randomly 1-4 lb on a regular basis. Unless you are up for a long period of time, I wouldn't sweat it. Hormones, salt intake, lack of water, etc, can easily put on lbs. Scale going up once is NBD!

Ya, too much change that a reason would be good to figure out.

Obviously not all fat, if any.

In heat more sweating, or sweating more in AC anyway?

Cardio increase?

Change to lifting routine, new rep/set ranges, new muscles?

Meds or diet change?

Neighborhood dog barking at night keeping you up and stressing you out nightly?

That's what I wondered about when it first starting going up, if getting hotter and sweating, or maybe not to sweating yet because body is keeping cool with greater blood flow.

That's one reason for inflated HR during a workout - body just pumping more blood for cooling effect, and when using more surface vessels to aid, that requires more blood volume. Most people gain that water weight as the do more in the summer.

Recent ride I lost something like 6 lbs while drinking 9 lbs of water. And that was not 6 lbs of fat or carbs burned up - 1 lb at most, so rest was literally just water weight from sweating, and lots of that from blood volume.
In fact it's usually found that if you lose 5% of bodyweight by that method for endurance cardio - you start impacting performance badly because now heart is pushing really thick blood, impeding oxygen flow.

So there is a lot of weight in blood volume. May not be sweating to prove it if merely moving it more is keeping you cool enough.

I am still wondering why such big jumps of water weight though, unless your cardio has increased too, or perhaps fewer but longer sessions, or C25K is running more often than walking, or more circuit style lifting instead of normal strength style, ect.

So I agree, not normal to be gaining like that, and while it still can't be fat that fast - unless you somehow think you are eating about 500 cal above true maintenance - still wonder why the body would be doing that?

Any meds with side-effects?

How long do you tent to retain water from TOM? It looks like you gained the week of and the next week (3&4), then a big whoosh on Week 5? Same thing on Week 6&7 although the loss was not as much the following week. You also upped the calories that week higher than the prior two weeks. Maybe some of that is water weight from the increased in cals. Does that make sense? I'm no expert as I'm just starting my journey but maybe your sweet spot is somewhere 2450-2550ish. I'm just speculating here, maybe someone with more experience will chime in later.