ECU reset now car is 'boged' down whilst accelerating

Ok so my indicator finally stopped working on my r reg (97-98) fiesta, I purchased a new switch assembly disconnected the battery whilst changing it and refitted the new one, the indicator now works. However we now arrive at the problem, whilst accelerating (in 1st I can accelerate up to 15mph then the car sort of judders and refuses to accelerate faster then the current speed of 15mph even when I put my footdown more, this occurs whilst in second at 25mph and third at 40mph). I no I shouldn't really be 'ragging' it but sometimes you need that bit of acceleration and it being a 1.25 it doesn't have that much to start with. The problem seemed to come when I disconnected the negative on the battery as before I did this it was fine. I know it would have reset the ecu as I had to reprogram my radio etc in just wondering how to get around this problem as when I hit the previously stated speeds the car literally feels like somebody has started braking and will decelerate. Thanks