Limping 10 year old

So my dog is a 10 year old mixed breed (about 30 pounds) a few months ago she started limping after walks or when she gets up in the morning. She runs fine and has a great time when she’s outside but about an hour after play she’s limping on her back left leg. I’m terrified to find out she’s sick. My only hope is that it’s been going on for a few months and hasn’t progressed so I’m hoping it’s just arthritis and the vet can give her anti-inflammatories. We’re saving up to take her now but has anyone experienced this? My dogs are my babies