As Mike said. If it hurts, get it looked at.
You say nothing has changed on the bike. Has any thing happened off the bike.
I hurt my back skating one winter. (i.e. fell). The back was quickly healed. About a month later my knee started to give me problems if I rode for 2 or more hours. I rested it. Did not fix it. So I got help. The physio quickly spotted that my hips were tight and uncomfortable and that i must have hit them hard at some time recently. The fall skating had put tension in my hips and this eventually worked through to my knee. They sorted it out quickly.
Vintage content. Non. Sorry.
Martin Coopland, Scotland, http://www.BatesBicycles.com