ok you guys theres something wrong with my bike and i dont know what...its a hybrid bike with 700c tires that are thinner than mountain bike tires but thicker than road bike tires...

the bike has been really easy to ride and now all of a sudden im on first gear on both the mid and rear derailer and its really difficult to ride...i get tired fast, and pedaling on 7th gear is so hard now...the breakes arent rubbing on the rim; the tires are straight with no spokje problems....only thing i see is that the rear tires tread has worn down and its now smooth like a road bike...but both tires have full air so i dont know if this could be the problem..