Welcome to Just Answer. How long ago were her anal glands checked? Is she licking back there more? Any discharge from her vulva?
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.
Anal gland expressions don't take any great skill to do! They just can be a bit gross and very smelly.
Here is a very good link talking about them and showing a diagram how to do it.
Honestly, there are a dozen or so you-tube videos showing how to do it. Please search it on there and watch a couple to see what it entails.