Why did you go for the scan (the one that originally located fluid in the Pouch of Douglas?) Was if for pain? Fluid can be in that location for various reasons, so to try to guess what the doctor is thinking, you need to consider what brought you in for the scan. You were treated for ten days for what condition, just the fluid? (Or was it for something like a ruptured cyst?)
Candidas is not unusual to find in the vagina, and often doesn't cause any problems. Did the doctor indicate any link between the candida and the fluid in the Pouch of Douglas?
And, you are asking about pelvic inflammatory disease. Did the doctor say this was what he or she was concerned about? It seems like PID would be revealed by some other marker besides just fluid in the Pouch of Douglas.
The main thing is, check with your doctor to find out what direction he or she is going with your diagnosis. There are too many things that can result in a little fluid in the Pouch of Douglas to be thinking it is only possible for it to be from PID. Google "what causes there to be fluid in the Pouch of Douglas?" and you'll see a lot of possibilities. And talk to your doctor.
Anniebrooke asks good questions and we'll look forward to your answers. To follow up on her response, yes, your original symptoms for why you had the scan would be most helpful. Some women get fluid at various times in their cycle such as near ovulation and others have as a secondary sign of something else. PID is one of the reasons women get this fluid in the pouch. Concerns about PID would be relevant especially if you have any other symptoms such as pain in your lower abdomen or pelvis, abnormal and heavy vaginal discharge with a heavy odor (which is not candida which usually does not have an odor and has itching involved, clumpy discharge and is very common as anniebrooke says), unusual bleeding such as in between cycles, painful intercourse, fever, difficulty urinating. https://www.mayoclinic.org/diseases-conditions/pelvic-inflammatory-disease/symptoms-causes/syc-20352594
Here's an article from a reliable source on fluid in the POD. https://radiopaedia.org/articles/differential-diagnosis-of-free-fluid-in-cul-de-sac-pouch-of-douglas?lang=us It does give a wide range of possibilities and I'm sure some of them would be most unwelcome. But from this position, we can't really tell you much. So, it's, as Anniebrooke is saying, imperative to see your doctor. This could really be nothing though, so keep that in mind.
What did they treat you with for the first time you had fluid? Some medications such as antibiotics are often the cause of candida or yeast infections. Basically they wipe out good bacteria that balances bad and makes conditions just right for yeast to grow. Is your doctor treating the yeast infection now?