How serious is this sudden swollen thigh?

Swelling can be anything from an abcess to a mast cell tumor.

Mast cell tumors are among the most common tumors in dogs and are the most common type of skin cancer found in dogs. Approximately 1/3 of all tumors in dogs are skin tumors, and up to 20% of those are mast cell tumors. The most common location to find mast cell tumors is, by far, the skin, followed by the spleen, liver, and bone marrow. Approximately half of all cutaneous (skin) MCT?s are found on the body proper, another 40% on the extremities (most frequently the hind limbs), and the remainder on the head or neck. Approximately 11% occur in more than one location.

Regardless of how your dog is acting now, something that is not right is not right, and should be seen by your vet to rule out the more seriouse possibilitys, or treat them promptly.

Good luck.