Could be a knock sensor or a loss of air inside the air box/tube that goes to the intake manifold. I would have the check engine light checked; just because it isnt on doesn't mean there isn't a code in the system. Most of the errors would give you a check engine light, so it might just be a matter of time till it comes on.
My Subaru had a faulty knock sensor => loss of power when accelerating
Mazda had a break in air duct => loss of power when accelerating