Could be many things. Get yourself a cup of tea and read my incomplete suspension setup sticky
Here area few pointers:
First check tyres. Frre to fix and common problem. Try 35 front 33 rear as a start.
Next check toe at your local garage. Lowering the car will make your rear toe in and your front toe out. Really lowering it will make the handling very twitchy if this isn't corrected. Concensus of opinon, though not necessarily perfect depending on driver, is 0deg toe all round for a road car.
When you do the one above, take it for a more expensive alignment instead and get your camber checked. I would advise max front of -1.5 and max rear of -1.0 for a raod car. Any more could again make the car twitchy under braking. If its more you might want to consider camber bolts or top mounts.
Another possibility is something is not tightened correctly or has failed. Again, a full alignment is a good place to start in diagnosing this.