I see you are on insulin, so I can't offer any input. Hopefully an insulin user will come along and help, but it may help them if you explain which insulin regime you are on, and when these 2 tests were taken (before or after meals)
Well it's a big drop but it's not impossible - would be dependent on many things including medication type and dosage, activity levels what you've eaten (or not).
I don't use insulin, so hopefully some type 2's who use insulin will drop by to answer any other questions you have, also on how to get signed up for classes.
I have had similar happen a few times not very pleasant makes me feel bad for a while I treated as a hypo but be careful as I found it at times would rebound back to high and then drop low again but would even out again after a few times.