Usually when the pouch of douglass had fluid in it, it is because of a
ruptured ovarian cyst. This could be a simple follicle or a cyst, but either way, after it ruptures, it generally resolves on its own, and has no consequence and is nothing to worry about at all. At certain times in the cycle, this is considered a normal finding.
Sometimes, like when evaluating an ectopic
pregnancy, it can be a concern, but it doesn't sound like that is your problem. There are many possible causes for the change in your flow, but if it comes on time every month, we are usually not concerned.
however, if it is extremely heavy or painful, there could be many possible causes. These could include infections,
endometriosis, polyps, or numerous other possibilities that your doctor can help you figure out.
Please feel free to ask me if you have any more questions.