Dog lay down in the middle of her walk, do I need to take her to the vet?

My 9-10yo mixed breed dog (maybe wolfhound cross) usually loves her walks and runs in and out of the water at the beach. We also do hour long walks in the bush. Today after 15mins she lay down and didn't want to walk further. So I sat with her and patted her. Now we're home she's moping around but not going outside. The only change is I fed her some kangaroo bones and beef bones (uncooked) the last few days and she hasn't been doing bigger poos and sometimes no poos. Is something wrong with her? Should I take her to the vet or just monitor her for a few days? She just went to the vet for a check up a month ago and all clear, just got her vaccinations and another set of injections for arthritis. Thanks so much