If you are were just cruising down the highway in 5th gear and all of a sudden you started slowing down then your engine is disconnected with your rear axle somewhere. Could be that your pressure plate and clutch disc is not making contact with your flywheel, or for some reason your transmission went into neutral, or a universal snapped. If your car started slowing down when you downshifted into 4th then your transmission may just me in neutral and your trouble is a faulty shifter leading you to believe you are in different gears but you are really still in neutral. If you did not hear any strange noise from the car when it started slowing down then the shifter would be the first thing I checked. If the u-joint snapped, the driveshaft would make an awful noise and be dragging on the pavement. The clutch arm could have come off the ball socket when you down shifted and the transmission is in neutral all the time. The shifter could just be very loose with maybe a missing bolt or two leading you to believe that you are down shifting or going into first when your transmission is just staying in neutral. Good luck