A lot of times, the bad smell from back there is due to the anal glands (even despite checking them 4 months ago). Some dogs will randomly express their anal glands on their own, they don't have any infection / trauam / blockage. The anal gland material is extremely foul smelling! IF it is expressed on its own, a lot of times they will then groom themselves / lick that area. She may be expressing them a little by licking them.

If there is no discharge or areas of redness back there, then the anal glands are most suspicious.

Strong, fishy smell - that screams anal gland issues. It could be from the perivaginal area, but usually there is redness and some wetness around that area. The best thing to do for anal gland issues like this is to have them expressed monthly (maybe a little more or less depending on a case by case basis). At this point, some owners will learn how to express them on their own. Your vet can show you how.