Any and all concerns about medical issues can and should be directed at your doctor! They're the only ones with enough info and qualifications to comment upon this.
Hip growth as a response to feminizing HRT can and does happen in younger patients. Hips don't set completely until the early 20s, so if you're starting HRT at or before then, you can experience both hip growth and your hips shifting in position. Heating pads and anti-inflammatories might help. Be sure not to push yourself too much if you're in pain, and as I already mentioned talk with your doctor if you're worried about the pain level.