Honestly, I would say take a HPT if you think there is even the slightest chance that you may be pregnant(ie: you've had sex in the last two weeks or so). This will get you into the doctor sooner for prenatal care sooner if you are pregnant. If it comes out negative, wait a week and test again.
My personal opinion on the discharge is that it may be fertile cervical mucus. This means that you will be ovulating very soon and then in about two weeks (10-16 days) you would get your period (or if you timed sex correctly, you'd be pregnant). Is it watery or looks like egg whites? That type of discharge means you are fertile. Go to www.fertilityfriend.com and read thier online turtorials. It will help you to understand your cycle.
I'm not sure what to tell you about the decreased supply. Is your daughter nursing for longer? Does she seem hungry even after feeding on both sides? Are you pumping and this is how you know? She is older and now starting to eat more table food. This may be the cause for the decrease.
The fullness of your breasts, if you are about to get your first pp period is caused by your pre-period hormones which are very, very similar to what you feel once implantation happens (7-10 days after you concieve). There is no way to tell if that is a pregnancy symptom or not. Only an HPT will know (or a blood test by your doctor).
Good luck!