When should you use a full stop in the middle of a sentence? When you’ve cut a word short and the last letter remaining is not the original one. For example:

January > Jan.   (the y is cut off)
et cetera > etc.   (the a is cut off)
id est (Lat. in other words) > i.e.   (the d and t are both cut off)
Latin > Lat.   (the n is cut off… etc.)
French > Fr.
company > co.
Professor > Prof.
Incorporated > Inc.

A full stop should not be used if the abbreviation does includes the original final letter:
Mister – Mr 
Missus – Mrs
Madame > Mme
Doctor – Dr
Limited – Ltd
Brothers > Bros