When I'm riding my bike at a high speed it feels like if I'm shifting gears when I'm not. Can anyone explain why this is happening?
If you want a specific answer, ask a complete question. Does it feel as though it's shifting UP or DOWN? More speed increases wind resistance - which makes it more difficult to pedal. It could, in fact be shifting gears on it's own. In which case...WHAT SETUP DO YOU HAVE? Is it maybe an 8 speed internal hub? Or maybe a typical 21 speed bike? (3 X 7 = 21) Twist grip shifters? Quick-Fire trigger shifters? Shimano STI shifters as found on a road bike? WHAT? How old is the bike? New bikes go through a settling in process. In which case, shift & brake cables may need to be readjusted. The derailleurs may need to be tweaked a little. Spoke tension on the wheels should also be checked. In short...when's the last time the bike had a TUNE-UP at a REAL bicycle shop? If ever? If purchased at a discount store, they do NOT service what they sell. If purchased at a bike shop, they all toss in a FREE after-the-sale "service warranty" good for at least one free tune-up as the bike gets settled in. The more information you provide, the better the answers might be. Question incomplete.
If you have a wornout chain or cog, the chain can jump a tooth while you are pedaling. You'll feel the skip but it won't actually shift to a different gear. Have a real bike mechanic look at it.
Who knows? No idea what sort of bike you have or how well it has been maintained. So, please tell more about what bike you have, its history, how well you have maintained it. A generic "bike" that has problems could have hundreds of possible solutions to those problems. Want a solution? fully describe the problem
I'm not sure if you're experiencing auto up shifting. Here's what the late Sheldon Brown had to say about it and how to fix the problem. https://www.sheldonbrown.com/autoshift.html
If your bike has a derailleur, then check for a tight link by slowly backpedaling and watching the chain as it goes through the jockey pulleys. If you've got a tight/frozen link, it'll cause the derailleur to 'jump' when it goes through. That 'jump' is what you're feeling do a half-shift. Free up the stiff link or get a new chain to fix the problem.
sounds like ghost shifting. take it to a bike shop for an adjustment and inspection
Does the bike shift automatically? If not properly adjusted your chain can occasionally jump gears of its own accord. It may just be that you're adjusting to a different speed and cadence.
It's an automatic
It's possible you are mentally retarded and unaware of how the gears work. Of course, I can't possibly know because I've never met you and I don't care to.