It's impossible to diagnose anything over the internet.  No, it's not normal, as such, but don't panic. 

Dogs do hurt themselves from time to time.  Dogs with three legs may struggle more with minor injuries than their four legged buddies.  It could be anything from a pulled muscle to arthritis, infection, rheumatism, cancer, back pain, kidney disease, or nothing much at all.   Has she put on much weight?  If so, that's another possibility; she may just have reached that 'tipping point' where they go from being able to cope with the extra weight plus being a tripod, to NOT being able to cope.  Sid gets very stiff if he puts on a pound or two extra.

Bottom line - if she's struggling, I'd take her along to the vet for a checkup.  He - or she - will be able to give you an idea what's wrong and offer some treatment options. 

As the others said, it's best to take her in to the vet. I was terrified to take our Abby in when she was suddenly lame on her lone front leg. I was afraid to go for fear he'd tell me the cancer had spread. But it was a sprained toe. Had myself all worked up for a hurt toe. (Granted - a hurt toe is a big deal when you've only got one front leg though.)

Hopefully it's nothing serious that a few days of rest won't take care of. Even if it is something more serious - it's always best to know because then you can come up with a plan.

Hang in there.

Jackie, Angel Abby's mom

Oooooh, please report back when you have some news - better, worse, a reason, a vet visit?  This is worrisome for sure, but without more time and a vet visit, who knows?  I'm relieved to hear she didn't have cancer before, as that would be my initial freak-out.  I hope you've got some NSAIDs for her or some other way to keep her comfy until there's a diagnosis.  Keep us posted!

One more thing: for acute injuries, use cold/cool compresses!  Frozen peas, anyone?  Until you know more, I wouldn't use heat to address her pain.

There was 1 night many years ago Jackson was in a lot of pain and couldn't move, step up, or walk.  We took him in and nothing was found, but NSAIDs were given.  We all decided he had some really bad bowel cramps from gas.  I hope it's that simple for your girl!

I hope she feels better soon.  

As far as the cold compresses, here's a tip I learned.  Take a household sponge and wet it.  Put it in a ziploc bag and put in freezer.  It makes a nice compress that doesn't make a mess when it thaws.  Good luck.