What type of insulin are you on? If you're on NPH, it probably peaked right after lunch, and if you went that low, then your dose was a bit too high and, as TF pointed out, it takes a while to tweak the dosages so that they're right for you. That could also happen on Regular, which has a faster-acting mechanism than NPH, but is still slower-acting than Novolog or Humalog.
I know those lows feel absolutely awful, but a low of 54 is, in the general scheme of things, not all that low. Yes, you must treat it, but you were aware of feeling lousy, you tested, found out you were too low, and acted accordingly. I've had blood sugar levels in the low 30's and still been able to recognize and treat them...NOT that I'm suggesting you wait until it goes that low, for sure. But you have good hypoglycemic awareness...the shaking, sweating, lousy feeling tells you that you need to test and treat, if low...and that's a good thing!
The other thing you could do to smooth things out would be to learn how to count carbs, work with your doc to figure out your insulin to carb ratio, and then bolus according to how many carbs you're going to be eating. It takes a bit of learning, but it's not that difficult and you'd probably be able to pick it up in a couple of weeks. Of course you'll still need some tweaking at first to get the insulin-to-carb ratio right.
The important thing is to remember that (unfortunately
) millions of people have learned how to do this, and we've all survived those lows. Try not to let this nuisance with GD destroy the enjoyment of your pregnancy.