Very stiff shifting

[troywk]

I was on a 40 minute round trip to the city yesterday in my Mini and on the way back my gear changes became very stiff and I had to use a lot more force than usual to put it into gear. It did this for about 10 minutes during which I tried to shift as little as possible in case I couldn't get it in gear. I got to a red light after a few miles of this and I left it in first with my foot on the clutch so I knew I could get going and when it turned green I started off as usual but when I went for second gear it was perfectly alright, like nothing was wrong in the first place. Does anyone know why it would've done this for a short period of time then stopped?

It was a warm day and the car had been running for a while already and I've never had anything like this before. My car is a 2002 Mini Cooper S. If you need any more information, just ask!