What can cause a 30 pound weight gain in 1 month when eating habbits have not changed?

Medications can cause weight gain. Not usually that rapidly tho. Are your legs swollen? The most common reason for rapid weight gain is that the person is retaining water. It can be all in your abdomen, so its less noticable unless you measure it or your clothes are really tight around your waist. Sometimes it can be your whole body.

Hi hippie chick is right. Here goes for the scary stuff. Cancer, intestinal, pituitary malfunctions. And on and on. What you need to do is call your doctor. Sudden significant weight increase or loss is serious. Kidney failure, congestive heart disease. All cause weight change because of water retention.

Because you have shown low blood sugar, you ought to be losing weight. Left untreated, hypoglycemia is deadly. Please see a physician right away.

As for blood tests. There are many kinds. This cannot be answered.

You didn't really give enough information for a good response, but here goes.
There are certain types of medications that a side effect is weight gain, psych meds and steroids are two examples. Also if you decreased exercise for any reason, also salt can cause water retention. I actually gain 25 pounds, all water weight, from an allergic reaction to one of my meds.
So a bit more information might help anyone else answering your question give you some better ideas as to what's going on with you and your weight gain..
Peace,
Sweet Hippie