I ride a 1970s steel-framed french racing bike. A couple of days ago I went over a small pothole whilst leaning on the front brakes at some speed, and although nothing happened it's since felt strange when riding in a straight line, almost tramlining. It's as if the front tyre is flat; there's an unnatural resistance to changing direction that there never used to be. Both tyres are pumped up, both wheels are true, and there's no discernible play anywhere that I can find.
I'd be grateful for any suggestions as to what this could be the cause of this!