If your injecting twice a day, you need to inject a good 30 mins before your meal (less if your pre-meal BG is low), you need to eat again 5-6 hours later and not miss out meals, you may also need a snack in between meals too.
On top of this you need to test your BG's before each meal, and at least 2 hours after each meal.
If your BG drops low then your body will produce its own glucose to help it rise and you will get a high BG reading a few hours afterwards, if you over treat a hypo or hyper then you can end up bouncing between highs and lows for a day or so.