Replying to Crf450r troubles

I know this isn't the place to ask help on dirt bikes but I don't know where else to ask and I know a lot of you ride. First off I have a crf450r and lately I wouldn't say I'm noticing a "loss of power" but more that every gear feels shorter. Let me explain: I have a small track in a field at my house, 2nd and 3rd gear is what I used to use to rip around the track. Now I end up using 3rd and 4th but I don't feel like I'm going any faster than I used too. I also feel like I fly through first gear like its not even there now. I used to be able to stay in first for awhile, now I'm shifting out of 1st within seconds. I re-shimmed the valves not too long ago but I haven't done the piston or rings in awhile. Any help would be appreciated. I honestly don't know that much about dirt bike engines, just letting you guys know. Thanks