Why did my mother's sugar level went low without taking an insulin.?

So my mother is a diabetic patient, she used to take two insulins per day as per doctors prescription. She visits the doctor after every 3 months or whenever the doctor tells her to visit [ which may be like next week or so, but since she was becoming alright doctor suggested her to visit after 3 months so he can reduce the amount of intake for her insulin].

Recently she went to visit the doctor after 3 months checkup, and the doctor gave her a new tablet that replaces the insulin. Doctor even said that she has cholestrol and some infection in her stomach so she has a vomiting sensation but no vomits. [It's a long story but I think this has nothing to do with it].

So yesterday afternoon she only took one tablet [ that replaces one of the insulin], she never took insulin it at night. From early morning her sugar level went low [ she was shivering, sweating and her heart was beating rapidly; father did not check her sugar level but gave her horlicks or milk and water].

Morning time I woke up and checked her level, it was around 42. She drank some milk and horlicks.

Afternoon it was 38, so she ate food just in case. Evening she was 35.

She still had that shivering and sweating.

Now my father took her to the hospital they can check with the doctor.

Why is this happening? I mean why did her sugar level went low when she didn't take insulin and still ate some candy or food.

I know I should wait for the result from the doctor but I'm just eager to know what others think.