Exhaustion

For a week out, yeah, that's 100% normal.

It took me almost 3 weeks before I got to the point where I had the energy to stay awake and active for more than 4 hours at a crack; until that point, it was more of a, "Wake up in the morning, take meds, putter around, deal with drains, putter around, 3 hour nap at noon, repeat until bed." thing.
Oddly, what helped for me with energy was Pokemon Go's release, which happened about a week and a half into my recovery.

That gave me 'incentive' to stop sulking on the couch and to get up and (slowly) walk around the immediate yard/block area, which did help with energy levels as well as helping to make me feel more 'normal'. I still play too, using it as my "excuse" to get out and take a 1-2 mile walk on my lunch break and/or in the evening.

I was back in the office at 3 weeks (worked at home week 2) and it was hard, for a few days, to get past the urge to go find somewhere to nap for an hour or so in the afternoon. By the 4th week that went away and I wasn't so easily wiped out all the time.

By the 5th week I had the energy levels to exercise like I used to, but didn't have full range of motion back in my arms yet so I spent a lot of time taking brisk walks.

I'm almost 3 months out now and, aside from residual swelling and a little 'tightness' if I extend my left arm (my left side always heals slower and is more swollen), I'm completely back to normal in terms of energy and activity levels without having any pain.

Should be noted that I did NOT have reconstruction or tissue expanders, so my heal times above are likely shorter than it would be for someone who had immediate reconstruction or who has tissue expanders.

I'm also used to being constantly active, alert, and awake, and wanted to get right back to that despite knowing that I needed to spend a few weeks resting and recovering from major surgery. I hated every second of having to rest and relax and not be up and moving around, but it's worth it as the more you overexert yourself the longer and worse your overall recovery will be.

Everyone is right: Your body does need to rest and you shouldn't push it--as annoyingly frustrating as that is.