So, I've only been playing for a month... mainly working on playing chords and transitioning between them. I noticed that whenever I start practice, my chord transitions are horrible. And it always takes me a good few minutes of practicing before I can start playing OK.

Is this normal? Is it just a beginner thing, or do good players also need to warmup first before they start playing well? It just feels a little dis-heartening when you finish a practice feeling that you've gotten something down pat, and then at the start of the next practice, you're all of a sudden unable to play as well.
warming up is something that ALL players do, and need to do.

what seems dis-heartening now becomes second nature to you after awhile.
That's why I practice at least an hour, so I'm at the top of my game for a while.
For some people it is that way. For others it isnt. I was kinda like that at the beginning. Just keep practicing and eventually you will get better.
I've been playing for about 18 months so I'm still a beginner but I wouldn't dream of playing without warming up.

I usually just run through chromatic scales for five or ten minutes then do what I want to do. Makes all the difference to me but as I've said I'm still learning too.
^ Chromatics and some barre chords would probably be your best bet for a warm up. That and anything that includes stretching ( maybe even stretches before you pick up the guitar ).