'01 SE, 5sp.

Today when I drove back from shopping, right out of the parking lot I couldn't put it into 3rd gear. The rest of the gears were fine and I had to do 1st - 2nd - 4th-5th to get home. After I shut off the engine, I stepped on the clutch and gave the shift lever a little pressure and it went into 3rd.

I restarted the car and for the rest of the day I drove around, the car is back to normal now.

What happened to the transmission? should I bring it into a dealer to check this out? it's got 45k miles and drive train still under warrenty.

By the way, how often should I change the manual transmission lubricant? I do maintenance myself and I haven't changed it so far.