Late complications of hip replacements have been reported. Swelling in the upper leg or thigh could be caused by your hip implant. Imaging studies with X-rays and/or CT scans may be needed to find out what's going on.
You'll need to see an orthopedic surgeon for an exam and testing. There have been rare cases of a fluid-filled mass in the thigh from debris wear after hip replacement.
Wear and tear on the polyethylene (plastic) liner or metal component parts can cause microscopic particles to build up in the joint or nearby soft tissues. Sometimes the patient reports pain or develops a limp, but there may not be any painful symptoms, just a mass such as you describe.
The physician will also rule out the possibility of a soft tissue or bone tumor, infection, fracture, or loose implant.