Hello Rachel,
Metformin is a good drug but it's not going to give you a hypo - at best it'll bring your numbers down by a point. That doesn't mean you can't experience symptoms that'll make you feel as you describe - even non diabetics can do this.
The trouble is if you've been running high numbers for a while, dropping down to 4.3 is going to make you feel odd. This happens to lots of diabetics when they start to get their numbers down. It doesn't last and as you stabilise it'll happen less often. Just remember to test yourself when you feel odd and you'll get an idea of the cause, because you can also get similar symptoms when you're too high.