Well, I'll try and put it into words how it was for me.
Low - snappy, tearful, angry, grumpy, feeling overwhelmed, so tired you want someone else to come in and take over, pissed off, sarcastic humour
Depressed - sitting wondering how your life got to be such a mess and realising you could sit there for hours, not wanting to see anyone or go out and finding ways to avoid people, ignoring the doorbell because your face is in such a mess from crying for a long time, being bouncy, cheery and helpful to all men and getting home and feeling the opposite like you've taken a coat off, thinking that if you do a,b and c you are bound to feel better
Does that help? I bought myself a book on amazon when I was facing the same questions. It is easy to read in short bursts (which is all the time you have) and it comprises case studies of some women whose stories are completely different. It might help you investigate further.
It is called At Home No One Hears You Scream (I'm a sucker for a great title!), Surviving Post Natal Depression, by Cara Aitken. If you have no luck and would like it, I could post mine to you (I'm pleased to say my veil lifted after much self examination a little bit of counselling and resistance to pills). I will also try to link something that might help sort out your feelings too. Remember, you might feel low, you might feel depressed, either way its normal and you will change again xxx