Can I ask what it was your friend noticed? I've got a similar situation, but if you don't want to that's fine 
I don't mind! I try to talk openly about this stuff whenever I can--and point it out whenever I see it in others--because if somebody had done that for me at
any point in my life? Things would be very different now. Besides, I've gotta talk to the doctor about it anyway, might as well get my story straight.
The thing about this friend is that she's
spookily dialed-in to my mental/physical/emotional state. She can usually tell if I'm getting sick, or if I have an emotional crisis creeping up, long before I notice it. (We have, by the way, never met face to face.) But she knows I won't take "you ~feel~ wrong to me" as a serious answer, so what she actually pointed out is that I've been tired lately. Gray. Enough that it's noticeable to anybody who knows me and cares to pay attention, even over text. With "lately" being (her words) 8 days out of 10 for the past two months at
least--yes, she keeps track of that kind of thing. Usually I'm a very talkative person, particularly with her, but every conversation I have with
anyone these days falls flat after the first sentence because I just have nothing to say. What's that Adventure Time quote? "I just been feeling kind of... gray, is all. Like my inside voice has been kinda quiet lately. Not a lot of instructions forthcoming, y'know?" It's like
that.
Once she pointed it out I could fill in the other blanks--being irritated at things that usually wouldn't bother me, avoiding phone calls/people/eating/[insert whatever basic-care thing you can think of here], sleep problems--both too much and too little, having to lower my expectations and fall back on old coping mechanisms in order to stay functional ("I will do ONE hard thing today and then I can sleep")... etc. Even stuff that's usually fun just gets wrapped in gray. You know I meant to do at least like 3 more posts on the Uniform Overanalysis thread over the hiatus? They're half typed up and everything! I just ended up... having nothing to give for them. I'd sit and stare at them for a while, then just wander off and do... nothing.
Most of it comes down to knowing myself and knowing what my personal warning signs are, how
I react when things aren't right in my head. I know by now (and shouldn't have needed reminding) that when my brain starts screwing with me I won't recognize it unless I keep an eye out for those warning signs. Because--moral of the story here--depression makes you stupid.
Really stupid. So: avoiding interacting with people that I
like--that's a bad sign. Feeling gray or foggy, like nothing is interesting, that's another one. Losing track of time, so I come-to hours later and wonder what I was doing that whole time--that's a very bad sign. Not having an appetite, or being hungry but not wanting to eat... I'm a former anorexic. That's a
horrifyingly bad sign. That's a "freak out, run in circles, eat like five cheeseburgers STAT" sign. And it should've freaked me out when I noticed it--it would've at any other time--but everything was too gray and it just didn't seem important. Again, depression? Turns otherwise reasonable people into complete idiots.