Dogs don’t show pain in the same way as humans. I had a dog with bone cancer and she was still running around like a pup. Only started limping when there was no bone left in the middle of that leg.
Unfortunately, slowing up and stiffness are both signs of pain. The stiffness is most likely arthritis, which restricts joint movement and causes pain, but it can be lower back pain. Stiffness was the first clear sign my boy had arthritis. Being slower to get up off their bed, up from lying down or up and down steps/stairs, not keen on sitting - preferring to lie down, less active generally around the house and garden, moving from bed to bed (finding it hard to settle in a comfortable spot) more sedate on walks, etc. Each dog will show signs in different ways. Also keep an eye out for uneven claw growth, as that can show how they’re favouring one leg or one side over the other.
The good news is that you are taking action as soon as you have noticed the stiffness, so there is a lot that can be done to help keep her comfortable and active. Often people say they are amazed at how it seems to take years off them when they start treatment. My boy is obviously a lot older, but he was running around in the park like a puppy, being really daft yesterday. Sadly, at this point, I have to intervene, as he will pay for it afterwards if he overdoes it, but he’s clearly happy and felt good enough to do it, which he wouldn’t have done this time last year. Since we’ve added in the supplements to his Rimadyl, he has started having his mad dashes in and out the back door at tea time again. We didn’t realise how hard he was finding things until the treatment plan began to take effect and he started being daft and cheeky again. We assumed being slow, less active and a bit grumpy was inevitable with old age. Since the treatment has started to work he is back to being himself. In fact, right now, he seems to be going through a second puppyhood - or maybe teens - as he’s started getting up to all manner of naughtiness he hasn’t done for years.