My puppy won't stand up and is lethargic... what's going on?

My puppy is 10 months old and he is pretty active. However, just this evening, he started acting very odd. He looked rather depressed, despite the fact that all day today he was happy. He couldn't climb up the stairs, where he was perched on the middle step. He wouldn't stand up when we interacted with him, and when we helped him up, he could barely stand properly on his back legs. He didn't yelp, whine, or act aggressive in any way. He had his tail between his legs but he could stand on his own. He refused to walk or go outside, and as soon as he was up, he lay back down, acting depressed again. We felt his tummy, looked at his gums, and felt the rest of his body for any other signs of trauma or swelling. His muscles around his abdomen and leg area were somewhat tense, which caused us to look at signs of bloat. He does not seem to have that. In fact, he seemed to have a good appetite, eating his food and drinking some water. We suspect that he has a muscle injury, but we are unsure, as he does not act as if he is in pain, other than the fact that he looks very depressed with his tail between his legs. He will not straighten his hind legs all the way when we massage the seemingly injured area. Is this a muscle injury, or is it something else? Should he be taken to the veterinarian, or just observed and rested for awhile?