I have had an early positive test before-a couple of days before my period was due. Some of the tests can detect a few days before your period is due now, most can a day or two before your period is due, and almost all can on the day you should have your period-unless your dates are off.
I "knew" as well way, way before I would have ever shown a positive test this time around...actually the night after conception. I woke up in the middle of the night with the "feeling" and suddenly became sick to my stomach and ran to the bathroom...and suddenly just "knew". Right on schedule, a couple days later, my breasts started feeling really sore (they never get sore during pms) and I kept telling my husband. I took a test about every 4-5 days until it finally showed up positive on Christmas-I believe it would have been a couple of days before my period was actually due as well. I knew it already, but just had to wait until there was enough of the hormone to detect it.
I wouldn't worry so much about a multiple just yet...different things, such as the time of day you take the test...how long it has been since you last urinated before taking the test, and other things like that will also determine exactly how much of the hormone is in your urine.
Go ahead and make your appointment, and when they do your labwork or first ultrasound, then any possibility of multiples will be found then.

When they test the hormone levels in your bloodwork-it is much more accurate as far as hormone levels than the urine tests for detecting multiples.
Oh...and congratulations.