Struggling to keep it in a straight line

[5.0 ED]

Had a flat on sat, drove a couple hundred metre's before realising, put spare on which is a standard 16" fairmont wheel when the rest au series 1 tickford 17's, and it bounced all over the road, i put on 2 new back tyres today and the same thing, darting all over the road, oversteering really bad, when i turn a corner its not responsive and then im oversteering really badly, only starts to do it at over about 70km's an hour under that driving around town it feels sort of normal. and its all in the rear end, kind of feels like i have 4 wheel steering, i had just put in new year balls joints and everything else was 100% and had a wheel alighment done at that time, i haven't had a chance to get under and have a look yet, but the question is could a flat tyre put the wheel alignment out? i let the steering wheel go and the car drives straight perfectly, and if i hold the steering wheel i feel like im constantly darting left and right.